A Technical Guide on Derailments (PDF)

CAMTECH/M/3 52 CHAPTER 4 PERMANENT WAY The track structure consists of following four main components : 1. Formation 2. Ballast 3. Sleepers and fastenings...

CAMTECH/M/3 52 CHAPTER 4 PERMANENT WAY The track structure consists of following four main components : 1. Formation 2. Ballast 3. Sleepers and fastenings 4. Rails These are required to be regularly and periodically checked by Assistant Engineer (IRPWM para 107), PWI (IRPWM para 123,124 & 139) as well as by P.Way Mistries, Mates and Keymen (IRPWM Part ‘C’). 4.1 FORMATION The railway track is laid over a formation prepared on soil (Fig. 4.1). The strength of formation depends upon the type of soil i.e. sandy, loam clay etc. and it serves the following purposes: distributes the weight of train, track and ballast over a wide area of natural ground facilitates good drainage and provides a smooth and regular surface on which the ballast and track can be laid The formation is affected by following factors: A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 53 Sudden subsidence of embankment Base failure Ballast puncturing due to heavy rains etc. Muddiness Fig. 4.1 B.G. FORMATION The inadequate care taken in maintaining the formation may cause derailments as failure of formation results in disturbance of track geometry. The steps to be taken for avoiding derailments on account of formation failure include measures to prevent sinking of track during diversions and use of new formations specially during rains. The geometry of track should be maintained as per laid down standards for passage of traffic at stipulated speeds. Where abnormal behaviour of formation or - supports is expected, the track geometry and packing/supports must be checked regularly or as often as warranted. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 54 4.2 BALLAST In the track geometry, ballast plays an important role. It absorbs noise, shocks, vibrations and distributes the load transmitted by the wheels over the formation. The ballast provides a flexible base to the track and controls the lateral and longitudinal movement of track. It keeps the track in position and at required level. If sufficient quantity of ballast is not available, track may get distorted andor buckled. The recommended ballast size is 50 mm. The profiles and minimum depths should be as given in Para. 263 of IRPWM. 4.2.1 Types of Ballast a) Cushion Ballast : The depth of ballast below the bottom of sleeper, normally measured under the rail seat, is termed as cushion ballast.(Fig. 4.2) Fig. 4.2 b) Crib Ballast : Ballast provided in between the sleepers is termed as crib ballast. (Fig. 4.3) A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 55 Fig. 4.3 c) Shoulder ballast : Ballast provided beyond the sleeper edge is called shoulder ballast.(Fig. 4.4) Fig. 4.4 At deep screening and relaying spots, the top table or gauge face of the rail gets smeared by ballast. This enhances friction at the flange contact area and encourages mounting of wheel specially in case of empty stock. The running surface of rail should therefore be maintained clear of ballast particles. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 56 4.2.2 Ballast Resistance The ballast plays an important role in absorbing shocks. The factors affecting the ballast resistance are : Ballast material Size Shape of ballast particle Ballast profile State of consolidation Type of sleeper 4.3 SLEEPERS AND FASTENINGS The sleepers and fastenings hold the rails within desired gauge parameters. If sleepers and fastenings fail, entire geometry of the track gets disturbed and the rails may shift from their due position. The loosening or failure of fastenings also indicate lateral distortion of rails under load. Thus they perform the following functions :  To hold the rails as per desired gauge limits  To transmit the load from rail to a wider area over the ballast.  To provide resilient support with ability to absorb high frequency vibrations  To provide lateral and longitudinal strength to the track  To permit rectification of track geometry  Be amenable to packing and retain it  To resist longitudinal creep of rail A TECHNICAL GUIDE ON DERAILMENTS April ‘98  CAMTECH/M/3 57  To resist overturning of rail Types of Sleepers are : 1. Wooden 2. CST 9 Cast Iron 3. Steel trough 4. Concrete The various fastenings used are bearing plates, fish plates, Screws, Spikes, Steel keys, cotters, Pandrol clips etc. The density and condition of sleepers should be maintained as prescribed in IRPWM. If ‘M’ is the Standard Single rail length in meters, the minimum sleeper densities as per IRPWM Para. 244 (4) are as follows : B.G. a) Traffic density more than 10 G.M.T. M+7 for Group A,B,C & D Routes M+4 for Group E Routes b) Traffic density less than 10 G.M.T. M+7 for Group A,B & C Routes M+4 for Group D & E Routes M.G.  M+7 for Q,R1 & R2 Groups; M+4 for R3; and M+3 for S Group of Routes. For prescribed sleeper spacing on various type of tracks, refer to IRPWM para. 244 (2). The orientation of sleepers to be checked by means of the gauge meant for this purpose. (All sleepers must be laid at right angles to the rail.) A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 58 4.4 RAILS The wear on rails play a vital role in increasing the mounting tendency of wheel on the rail table. The excessive wear may even result in rail fracture which may cause derailment. The types of rail wear (Fig. 4.5) are : A. Vertical B. Lateral C. Angular Fig. 4.5 Limits of Rail Wear The permissible limits of rail wear (IRPWM Para. 302 iii & iv) are given below : 4.4.1 Vertical wear Measure the rail height at the centre of rail (normally using callipers or profiling). A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 59 Gauge Rail Section Limit 60 KG/M 13 mm B.G. 52 KG/M 8 mm 90 R 5 mm M.G. 75 R 4.50 mm 60 R 3 mm 4.4.2 Lateral wear Section Gauge Category Limit of of track Lateral wear Curves B.G. A&B 8 mm C&D 10 mm M.G. Q&R 9 mm Straight B.G. A&B 6 mm C&D 8 mm M.G. Q 6 mm R 8 mm Note : The lateral wear should be measured 13 mm below the top surface of the rail. The maximum angular wear permitted is 250 on all tracks. The typical profile and main dimensions of a new 60 KG UIC rail section are shown in Fig. 4.6. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 60 60 KG/M RAIL SECTION (All dimensions are in mm) Fig. 4.6 If vertical wear is excessive, a deep flange may ride over the fish plate/distance block/check block and may damage the track components. Excessive lateral wear increases the play between the wheel set and the track which would contribute to increased oscillations and greater angularity of the flange during run. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 61 The angular wear is most critical. Angular wear is invariably encountered on the outer rails on sharp curves as well as on turn outs. If angular wear is excessive, the rail presents an inclined plain to the wheel on which the flange may slide upwards. 4.5 GAUGE The distance between the two running edges of left and right rails is known as Gauge (Fig. 4.8). It is 1676 mm on B.G. The irregularity in gauge leads to excessive sinusoidal motions of the vehicle leading to development of attack of wheel flange with the rail. If gauge is found less than 1676 mm, it is termed as Tight Gauge. If the gauge is more then 1676 mm, then it is termed as Slack Gauge. 4.5.1 Effect Of Tight Or Slack Gauge Due to slackness in gauge, play between flange and running edge of the rail increases. Thus excessive slackness further increases lateral oscillations, hunting, excessive flange forces and angularity due to which the wheel may drop. The main indication of slack gauge is that either one wheel remains on the track and the other drops inside the track or both wheels drop inside the track. 4.5.2 Tight Gauge Tight gauge increases the strain on track fastenings and creates a tendency for the wheel to lift on run. Due to tight gauge, the flange of the wheel starts grinding against the rail edge. This condition causes high flange forces to occur and the flange ultimately mounts over the rail. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 62 4.5.3 Causes Of Gauge Distortion The following are the major causes of gauge distortion : Worn out fastenings due to which track could not hold the correct gauge. Damaged sleepers and unserviceable sleepers due to which fastening become loose. Fastening not properly secured and becoming loose due to high speed vibrations. Missing fastenings i.e. keys, dogspikes etc. 4.5.4 MARKING STATIONS FOR TRACK MEASUREMENTS For investigating the cause of derailment, various measurements pertaining to track are required to be taken carefully. Before starting the measurements, point of mount or drop has to be ascertained jointly by nominated supervisors since the point of mount or drop is taken as reference point for many measurements (Fig. 4.7). From the point of mount/drop, stations are to be marked on the rail table at a interval of 3 meters on either side of the track up to a distance of 45 meters. Hence 15 such stations are to be marked if the cause of - derailment is normal. However if the cause is abnormal, as many as 30 stations at the rear of the point of mount/drop can be marked covering a distance of 90 meters if necessary. After completing this work, measurements with regard to track should be recorded from one end. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 63 Fig. 4.7 4.5.5 How To Measure Gauge The Gauge should be measured at stations already marked 3 meters apart with the help of Gauge-cum-level instrument. Before using this gauge, ensure its correctness as well as spirit level sensitivity. It must be ensured that both ends are properly touching the running edge of the rails during measurement. The tightest point obtained determines the correct point to test the gauge. Record the reading showing tight or slack gauge. The gauge is designed to measure slackness or tightness up to 20 mm. If tightness or slackness is found beyond measuring capability of the gauge, it should be measured by tape and recorded (Fig. 4.8). Gauge is measured 14 mm below the rail top table. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 64 Fig. 4.8 4.5.6 Permissible Gauge Tolerances As per Para. 224(v)of IRPWM, the tolerances for gauge are: TIGHT SLACK On straight (All Gauges) 3mm 6 mm On curves with radius: i) more than 400M on B.G. 275M on M.G. and 3mm 15 mm 175M on N.G. ii) less than 400M on B.G., 275M on M.G. and Nil 20 mm 175M on N.G. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 65 4.6 CROSS LEVEL The relative difference in the height of left and right rail at a given point on the track is known as Cross Level (on straight track). The fluctuating cross level differences in the track result in track Buckling (Para. 4.9) and help in developing undesirable oscillations on vehicles adversely affecting their stable running. 4.6.1 Effect of Variation in Cross Levels Due to variation in Cross Levels, either one wheel is above or below the plane of other wheels of the vehicle. If one rail is higher, the wheel on that spot will be on a higher plane and will cause off-loading of that wheel. On the other hand, if one rail is lower, the wheel on that spot will be on a lower plane and will cause off-loading of the opposite wheel. Thus variation in cross levels affects the stability. A uniform cross level difference does not matter much from the point of derailment. A cant deficiency up to 75 mm is normally permitted. If cross level differences up to this extent are uniformly available over long stretches in the track, it may not lead to unstable conditions. However the variations in cross level called Twist are very important from safety point of view. If the cross levels vary too frequently, the effect is the same as difference in camber of springs and this may lead to derailments. This effect is more pronounced in case of a four wheeler stock in which one of the wheel tends to float due to excessive twist and may get derailed. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 66 Cross level is an important factor to be considered in derailments and it should be recorded very carefully. Cross level should be recorded taking left rail as the reference rail i.e. recording whether the right rail is low or high. Normally cross levels are measured from the point of mount or drop up to 45 meters on either side. Where point of mount is not clear, 90 meters in rear and 45 meters ahead may be taken for measurement. The readings are essentially recorded from sleeper to sleeper. 4.6.2 How to Measure Cross Levels For measuring cross level, modified RDSO gauge is used which records Gauge as well as Cross Levels simultaneously. For measuring cross level, a good quality of spirit level is used and is kept at the centre of the gauge. Before using spirit level, the sensitivity should be checked i.e. whether the bubble is freely moving in the cage or not. In the gauge, space is provided for spirit level which slides freely on the gauge. The sliding portion is calibrated on either side with which the difference in cross level in mm can be read directly. If spirit level moves 10 mm to the right of the gauge, it indicates that right side rail is lower by 10 mm as compared to the left rail. If spirit level moves left side then this indicates that left rail is lower than right rail. For measuring the cross levels, always take left rail as reference rail and it should be clearly mentioned i.e. left rail is low or high. Though maximum permissible variations in cross level have not been laid down , a 13 mm variation in cross levels at stations 3 meter apart is considered limiting value. - Railway Board vide letter no 63/wg/tk/10 dated 10.11.64 has laid down following tolerances for B.G. (see Table 4.1) A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 67 TABLE 4.1 During During service maintenance Cross Level Variation not more Var. not more than 8 difference than 4mm mm. Twist on 3 Var. not more than Var. not more than metre base 2mm/metre 3mm/metre 4.7 TWIST Twist is rate of variation in cross level per meter. Normally twist is calculated for the stations marked on the track 3 meters apart from the reference point. If one rail is higher than the other at station No. 1 by 10 mm and the same rail is lower than the other at station No. 2 by 8 mm, then the twist between station No 1 & 2 (3 meters apart) will work out to be : (10 + 8)/3 = 6 mm per meter If one rail at station No. 1 is higher by 12 mm & on station No. 2, it is higher by 6 mm, the twist will be : (12-6)/3 = 2 mm per meter In finding out the real cause of derailment, the twist should be measured on the wheel base of the vehicle which derailed first since this is the twist which played effectively at the time of derailment. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 68 4.7.1 Effect of Twist Twist has an adverse effect on running and plays an important role in derailment. The reason is quite clear. If at station 1, left rail is higher, than the left wheel is off loaded. If at station 2, right side rail is higher, right side wheel is off loaded & left side wheel is loaded more. This will cause uneven loading of wheels and this condition is prone to derailment. 4.7.2 Calculating Effective Twist Let the wheel base of the first derailed vehicle is 4 meters. The twist will be calculated as under: Distance Cross Level Twist over Effective Change Twist from Ref. reading base of over 4 meters in point in mm. measurement wheel base mm per (meters) (mm/meter) mm meter 0 +5 - (+5)-(-6)=11 2.75 1 +7 2 (+7)-(+7)=0 0 2 0 7 (0)-(+2)=-2 0.5 3 +3 3 (+3)-(+10)=-7 1.75 4 -6 9 5 +7 13 6 +2 5 7 +10 12 The highest value of effective twist on 4 meter wheel base of the vehicle is between station 0 and station 4 above. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 69 4.7.3 Permissible Standards for Twist 1 Track Twist in mm/metre Category B.G. M.G. (3.6M chord) (2.74M chord)  A 0 to 5 0 to 3 B 5 to 7.5 1 to 2 C 7.5 to 10 2 to 3 4.8 TRACK ALIGNMENT AND UNEVENNESS The track should be in well aligned position for good comfort and safety and limit the chances of flange coming in contact with the rail. The flanges are actually the block stops for exceptional situations. This has been clearly highlighted by French Railways (Appendix „E‟). On encountering a track irregularity, the springs start oscillating and continue to offload the left and right wheels alternatively even after the irregularity is crossed. If the natural frequency of the vehicle happens to coincide with the exciting frequency, the oscillations can develop dangerous proportions causing derailment. The lateral alignment using 7.2 metre chord and vertical alignment i.e. Unevenness (using 3.6 metre chord for B.G. & 2.74 metre chord for M.G.) are to be maintained within following service 1 tolerances : Track Category Alignment (mm) Unevenness (mm) B.G. M.G. B.G. M.G. A 0 to 3 0 to 3 0 to 6 0 to 3 B 3 to 5 3 to 5 6 to 10 3 to 5 C >5 >5 >15 5 to 8  A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 70 1 Ministry of Railways, Government of India. Investigation of derailments. (Pune: Indian Railway Institute of Civil Engineering,1995), p. 22. 4.9 CREEP The longitudinal movement of rails in the track is called Creep. Creep on the rails vary from place to place. In some places it may be as much as few centimetres in a month whereas at some other location, it may be negligible. Due to creeping of rails, the openings in the rail joints at the point of creep gets widened. 4.9.1 Causes of Creep Many theories have been evolved to explain the reasons for creeping of rails in a track. Some are explained as below. A. Wave Theory According to this theory, a moving train sets up a wave motion in the resilient track portion of the rail. Immediately under the wheels of the train, the rails between adjacent sleepers get depressed or deflected somewhat due to vertical loading. The moving wheel pushes the elevated portion of the wave, which in turn forces the rail in the direction of traffic. The raised portion of the rail in front of the moving load is carried forward by the wheel and this helps in developing creep. The raised portion of the rail at the rear of the moving wheel then assumes its normal position. Along with the moving wheels of the train, the A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 71 depressions formed in the rail also moves with them. As soon as the loaded wheels pass over a depression, the portion of the rail springs back to its original level. In this way, the wave motion tends to move along the entire length forward with the moving train. Similarly the impact of several wheels at a rail joint also causes creep. The pitch and depth of the wave in the track however depends upon the following factors : Stiffness of the track Spacing of the sleepers Wheel base of the vehicles Weight of the rails quantity and quality of the ballast Standard of the maintenance of the track Track modulus of the material. To reduce the creep, it should be ensured that the track possesses the following qualities : Increased stiffness Stable soil for formation Angular ballast for better interlocking B. Change of Temperature Theory Development of creep rail may be caused by expansion/contraction of rails due to change in temperature. The change in temperature depends upon the range of temperature and the location of the track i.e.. whether the portion is exposed to sun or remains under shade. Creep is more rapid during hot A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 72 summer as compared to cold winters C. Unbalanced Traffic Theory According to this theory, if heavy traffic moves in one direction and light traffic moves in opposite direction predominantly on a single line track, unequal creeping occurs between the two directions. The net effect of creep in the direction of movement of heavier traffic is more. In case of double line tracks , the movement of trains is more or less unidirectional and as such creep develops in both the tracks in the direction of movements of trains. Besides the above theories, the creep may also develop due to the following factors : 1. Loose fittings of rails 2. Bad quality of sleepers 3. Bad alignment of curves 4. Too tight or too slack gauge 5. Gradient of the track 6. Life of the rails 7. Uneven spacing of the sleepers 8. High embankments. 9. Weight of the rails. In fact, it is very difficult to predict the magnitude of creep. Some times, only one of the rails may also start creeping in a direction opposite to the other rail A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 73 4.9.2 Effect of creep Though there are several serious effects of creep, the most serious is buckling of the track. If proper attention is not paid, a buckled track may easily derail a train. The other harmful effects are as under : The sleepers move out of square and position. The gauge and alignment of the track gets disturbed. The surface of the rails get disturbed considerably due to movement of sleepers from their packed loads. This causes uncomfortable riding. Rail joints at the starting points of the creep get opened out of their permissible limit considerably. The stresses develop in the fish plates and bolts. This may sometimes break the fish bolts and the rail ends gets battered badly. At the point where creep ends, the rail joint gets jammed and thus prevents free expansion of the rail. Points and crossings get disturbed and it becomes difficult to maintain the correct gauge and alignment. The movement of switches becomes difficult and even the interlocking arrangement is thrown out of gear. The expansion gaps become either too big or too short. Besides above defects, kinking of rails forcing the ballast and even smashing the fish plates and fish bolts are common A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 74 occurrences due to creep. 4.9.3 Measurement Of Creep The measurements of creep in a track may be done in the field as explained below : A. Fix two creep indication posts or unserviceable rail pieces on either side of the track in the formation such that their tops are about 25 mm above the rail level. B. Fasten a string to the tops of the posts such that it passes freely below the rail bottom. C. Make a chisel mark or a point mark on the side of bottom flange of the rail on either side of the track. Note down the time and date of marking. D. After a particular interval of time, measure the distance between the chisel marks and the string. This distance is the required amount of creep which has developed during the given time interval. 4.9.4 Remedial Steps against Creep The following precautions are required to be taken to prevent and reduce the creep of rails in the track : A. Keep the sleepers always well packed with heavy angular ballast and provide wide shoulders as to resist the A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 75 movement of the sleepers. B. Keep all the rail joints levelled so that there are less chances of impact by the moving wheels. C. Keep the fish bolts tight and oil them frequently so that the ends of the rails move freely under thermal stresses. D. Provide anti-creepers or anchors on the top of sound sleepers. If the creep still continues, the number of anchors may be increased per rail panel. 4.9.5 Track Maintenance against Excessive Creep The maximum permissible limit of creep is 150 mm. In case the creep exceeds this limit, the adjustments must be made as under : A. Measure the expansion gaps and check against the total amount of gap in the length which should be equal to the standard expansion gap required at the given temperature multiplied by the number of joints in the rail panel. B. Remove the keys and fish plates. Insert correct expansion liners before pulling the rail back with bars. C. Tighten up the fish plates and insert key properly. D. In case the total gap is more than the standard gap, the required closure rails may be used with a speed restriction of 30 Km/h till the gaps are removed. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 76 E. While adjusting the creep, the sleeper spacing should also be adjusted. 4.9.6 Track Lengths susceptible to Creep The locations in track more susceptible to creep are : Steel sleeper trackCST-9 sleeper track joining the wooden sleeper track. Long stretches on gradients having dips Approaches to major girder bridges or other stable structures. Approaches to level crossings and points & crossings. 4.10 BUCKLING OF TRACK Buckling of track occurs when very high compressive forces are created in the rails. A special watch should be kept at the junction of two stretches of track where one is liable to creep and the other is held against creep. Some of the examples are: Track laid on wooden sleepers with inadequate anchors and scanty ballast. Track laid on metal sleepers with loose keys butting against the track laid on new sleepers with tight A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 77 fastening. Track anchored with welded track. As one side of such a junction point is held firmly, the movement of rails due to creep from the other side is restricted resulting in heavy compressive forces being exerted. This will tend to buckle the track. Jammed rail joints are therefore an indication of the track being subjected to undue strain which may result in track buckling. 4.10.1 Conditions inducing Buckling The following conditions create high compressive forces in the rail: Inadequate expansion gaps Failure to counter act creep in time Non lubrication of rail joints Failure to remove rail closures from track 4.10.2 Precaution against Buckling A. Greasing of fish plates should be done before the hot weather sets in. B. The joints gape survey is done in the case of SWR and adjusted before the hot weather. A TECHNICAL GUIDE ON DERAILMENTS April ‘98 CAMTECH/M/3 78 C. Adequate precautions are taken to reduce creep. D. Over tightening of fish plate bolts is avoided - but they should be reasonably tight. Fig. 4.9 Shifting of Sleepers in Derailments due to Buckling It is observed in many derailments attributed to track buckling that a severe cut/dent mark occurs on the foot board brackets on the inner side of non-derailed coaches having passed the point of derailment. Corresponding grazing marks are also A TECHNICAL GUIDE ON DERAILMENTS April ‘98