Introduction to Railway Signalling PDF

  Introduction  to  Railway  Signalling         1.   Why  do  Railways  have  a  Signalling  System   There  are...

  Introduction  to  Railway  Signalling         1.   Why  do  Railways  have  a  Signalling  System   There  are  two  fundamental  physical  reasons  why  railway  signalling  systems  exist:     1.   Trains  are  guided  by  the  track  and  hence  have  to  be  routed  in  such  a  way  as  to  avoid  collisions  with   one  another   2.   Trains  cannot  stop  within  the  distance  that  the  driver  can  see  –  they  need  to  have  prior  warning  of   the  need  to  slow  down  and  stop  ahead   2.   Signalling  System  Basics     2.1   Basic  Principles     The  basis  principle  underpinning  all  UK  signalling  systems  is  the  Block  System.     Each  line  is  divided  into  Block  Sections,  and  except  in  particular  circumstances  only  one  train  is  permitted  to   be  in  each  block  section  at  any  one  time.       A  signal  is  provided  at  the  start  and  end  of  each  block  section  to  allow  the  train  to  enter  and  exit  the  block.     2.2   Functions  of  a  Signalling  System     Based  upon  this  simple  principle,  signalling  systems  have  evolved  to  provide  the  following  key  functions:     Safety  Functions:       To  prevent  trains  taking  conflicting  routes     To  maintain  a  safe  separation  distance  between  trains     To  protect  trains  from  driver  malfunction  (incapacity  /  inattention  /  misjudgement)     To  ensure  trains  do  not  exceed  their  permitted  speed     Non  Safety  Functions:       To  maximise  the  use  of  the  track     To  route  trains  automatically  and  regulate  their  flow     To  provide  data    on  train  running  for  passenger  information  purposes     Not  all  functions  are  provided  at  all  locations     3.   Generations  of  Signalling  Systems     Within  the  UK  there  are  three  generations  of  signalling  system  in  use  or  planned:       Mechanical  Signalling  -­  is  basically  1860’s  technology,  but  there  is  much  of  it  still  in  use  especially  on   regional  lines     Multiple  Aspect  Signalling  –  is  the  predominant  generation  of  signalling  in  use,  and  covers  virtually   all  main  lines  in  the  UK.  Its  use  began  in  the  1930’s  and  after  a  pause  for  World  War  2  was  rolled  out   over  much  of  the  network  in  the  1960s,  70s  and  80’s.  It  is  still  being  installed,  albeit  with  more   modern  technology.   Prepared  by  Neil  Porter   Page  1  of  8   Introduction  to  Railway  Signalling       v1.0_Additional  reading.docx       Introduction  to  Railway  Signalling                   Cab  Signalling  –  Signals  are  dispensed  with  and  instructions  to  the  driver  are  displayed  in  his  cab.   This  is  the  next  generation  of  signalling  based  on  ETCS.     Hybrid  systems  also  exist.     In  the  case  of  mechanical  and  multiple  aspect  signalling,  the  driver  is  required  to  “know”  the  route  he  is   travelling  and  therefore  the  speed  restrictions  (including  at  junctions),  and  the  signal  aspects  provide  him   with  specific  routing  information.   4.   Mechanical  Signalling     Mechanical  signalling  developed  in  the  mid  1800’s.  The  signalman  operates  large  mechanical  levers  in  a   small  wayside  signalbox  to  change  the  state  of  individual  points  and  signals.  The  lever  frame  in  which  the   levers  are  mounted  incorporates  mechanical  interlocking,  such  that  individual  levers  can  only  be  operated   when  other  levers  are  in  the  correct  position.  The  levers  are  connected  to  the  signals  and  points  by  means  of   mechanical  connections  of  rods  or  wires.       Signals  are  of  the  semaphore  type,  and  consist  of  two  key  types:     Stop  signals  –  the  driver  has  to  stop  at  these  if  the  signal  arm  is  horizontal  (at  danger).  If  it   is  raised  (or  sometimes  lowered)  by  approx  60  degrees,  then  this  gives  the  driver   permission  to  proceed  beyond  the  signal  (into  the  next  block  section)         Distant  signals  –  if  the  arm  of  these  is  horizontal,  it  advises  the  driver  that  the  next  stop   signal  is  at  danger,  and  he  must  slow  to  stop  at  it.  If  it  is  raised  (or  sometimes  lowered)  by   approx  60  degrees,  then  it  advises  the  driver  that  all  the  stop  signals  ahead  controlled  from   this  signalbox  are  clear,  and  that  he  can  proceed  at  the  speed  of  the  line.  Distant  signals  are   positioned  at  least  braking  distance  from  the  next  stop  signal.     A  typical  mechanically  signalled  layout  is  illustrated  below:               At  diverging  junctions,  indications  of  route  to  the  driver  are  provided  by  mounting  multiple  stop  arms  on  a   single  post.           Prepared  by  Neil  Porter   Page  2  of  8   Introduction  to  Railway  Signalling       v1.0_Additional  reading.docx       Introduction  to  Railway  Signalling                 5.   Multiple  Aspect  Signalling     Multiple  aspect  signalling  was  a  development  designed  both  to  increase  the  train  carrying  capacity  of  the   track  and  to  enable  a  single  signaller  to  control  a  much  larger  geographical  area,  since  he  is  not  restricted  by   mechanical  rods  and  wires  to  be  in  close  proximity  to  the  signals  and  points.     The  functional  of  mechanical  stop  and  distant  signals  are  combined  into  a  single  colour  light  signal  with  three   or  four  aspects.       A  red  light  requires  the  driver  to  stop     A  single  yellow  advises  him  that  the  next  signal  is  at  red,  and  therefore  to  slow   appropriately     A  double  yellow  advises  him  that  the  next  signal  is  at  yellow,  and  hence  the  one   beyond  that  is  at  red,  and  therefore  to  slow  appropriately     A  green  advises  him  to  proceed  at  linespeed         A  typical  sequence  of  aspects  as  trains  proceed  along  the  line  is  illustrated  below:             At  diverging  junctions,  indications  of  route  to  the  driver  are  provided  by  adding  junction  indicators  above  the   signal,  which  is  lit  in  addition  to  the  main  aspect  when  a  diverging  route  is  set.                                         Prepared  by  Neil  Porter   Page  3  of  8   Introduction  to  Railway  Signalling       v1.0_Additional  reading.docx       Introduction  to  Railway  Signalling                 6.   Components  of  a  Signalling  System     The  following  section  highlights  the  key  elements  of  the  signalling  system.     6.1   Train  detection     The  function  of  a  train  detection  sub  system  is  to  determine  if  a  particular  section  of  track  is  occupied  by  a   train.       6.1.1   Manual  Detection   With  the  original  mechanical  signalling  systems  the  only  form  of  train  detection  was  manual  observation  by   the  signaller  looking  out  of  the  signalbox  window.       6.1.2   Track  Circuits     To  protect  against  human  error,  track  circuits  were  developed,  which  use  insulated  sections  of  the  rails  as  an   electrical  circuit,  which  the  wheels  of  a  train  shunts  as  it  enters  the  section.     Conceptually  a  track  circuit  is  represented  thus:           In  the  simplest  form  the  transmitter  is  a  battery  and  the  detector  is  an  electro-­mechanical  relay.     Many  much  more  sophisticated  types  exist  using  coded  audio  signals  and  frequency  shift  keying  (FSK),   which  were  developed  to  provide  immunity  from  EMI  generated  by  electric  trains.     The  track  circuit  illustrates  the  key  principle  of  “Fail  Safe”  applied  to  all  traditional  signalling  equipment,  in   that  any  break  in  the  circuit  between  the  transmitter  and  the  receiver  has  the  same  functional  effect  as  a  train   shunting  the  rails,  and  hence  the  system  fails  to  a  safe  state.       In  Multiple  Aspect  Signalling  installations,  large  numbers  of  individual  track  circuits  cover  the  entire  track   layout  to  provide  complete  train  detection.             Prepared  by  Neil  Porter   Page  4  of  8   Introduction  to  Railway  Signalling       v1.0_Additional  reading.docx       Introduction  to  Railway  Signalling                 6.1.3   Axle  Counters     Axle  counters  were  developed  to  avoid  the  need  to  electrically   insulate  sections  of  rail  (as  required  for  track  circuits)  and  to   provide  a  greater  degree  of  EMI  immunity  from  electric  trains.       They  operate  simply  by  counting  the  axles  of  a  train  entering   and  leaving  a  section  of  track.  If  the  section  is  initially  clear,   then  any  net  number  of  axles  in  the  section  implies  that  the   track  section  is  occupied.     Axle  counters  are  becoming  the  preferred  means  of  train   detection.  A  typical  counting  head  is  illustrated.         6.2   Point  operating  mechanisms     Point  operating  mechanisms  provide  3  key  functions:     To  move  a  particular  set  of  points  (switches  in  civil  engineering  terms)  from  one  position  to  another     To  physically  lock  them  in  that  position,  to  provide  security  for  the  passage  of  trains     To  detect  that  they  are  both  in  the  correct  position  and  locked.     6.2.1   Mechanical  Points     In  mechanical  signalling,  points  were  operated  and  locked  by  means  of  galvanised  steel  rodding  attached  to   the  levers  of  a  mechanical  signalbox,  such  that  the  signaller  provides  the  motive  power  for  the  movement.     Detection  was  also  provided  by  mechanical  means  with  arrangements  of  rods  and  sliders.     6.2.2   Point  Machines     A  large  number  of  different  types  of  point  machine   exist,  but  they  all  combine  the  functions  of  control,   locking  and  detection.  Usually  the  motive  power  is   electrical,  but  occasionally  piped  compressed  air.       A  typical  point  machine  is  illustrated.           6.3   Signals     Signals  fall  into  2  key  types,  semaphore  and  colour  light,  as  discussed  in  sections  4  and  5.     There  are  many  further  sub  types,  conveying  particular  messages  to  the  driver.  ,     Colour  light  signals  have  until  recently  been  based  upon  incandescent  bulbs  and  lens  systems,  however   these  are  rapidly  giving  way  to  arrays  of  LEDs           Prepared  by  Neil  Porter   Page  5  of  8   Introduction  to  Railway  Signalling       v1.0_Additional  reading.docx       Introduction  to  Railway  Signalling                 6.4   Interlocking     The  functions  of  the  interlocking  are:       To  maintain  a  current  record  of  the  position  of  every  train  in  the  control  area     To  maintain  a  current  record  of  the  status  of  all  signalling  outputs  (position  of  points  /  aspects   displayed  by  signals  etc)     To  process  the  signaller’s  input  requests  to  set  a  particular  route  or  swing  a  set  of  points  etc,  and  to   determine  if  these  requests  are  safe  given  the  current  situation.  If  so  to  control  signalling  outputs  in   accord  with  the  signaller’s  request.     6.4.1   Mechanical  Interlocking     Mechanical  interlocking  in  a  mechanical  signalbox  consists  of  a  series  of  steel  bars  and  tappets  connected  to   the  levers,  which  prevent  unsafe  combinations  of  levers  being  pulled  by  the  signaller.  These  are  still  common   on  rural  lines,  but  rely  for  some  of  their  safety  integrity  on  the  signaller.     6.4.2   Relay  based  Interlocking     These  were  developed  from  the  1930’s  onwards,  and  reached  their  peak  of  development  in  the  1960’s  /  70’s.   Here  the  interlocking  functions  are  provided  by  very  large  numbers  of  electro-­mechanical  relays,  drawn  from   a  range  of  relays  specifically  designed  to  operate  in  a  highly  reliable  fail  safe  nature.     The  required  functionality  at  a  particular  location  is  literally  hard  wired  between  the  relays.     6.4.3   Computer  based  Interlocking     There  are  a  number  of  types  in  operation  in  the  UK,  but  the  most  widely  used  is  known  as  SSI  (Solid  State   Interlocking)  and  was  developed  by  BR  in  the  1980’s.  It  is  based  upon  an  8  bit  microprocessor,  and  achieves   its  safety  integrity  by  operating  3  processors  in  parallel,  of  which  at  least  2  have  to  agree  on  a  change  of   output  before  it  is  put  into  effect.     The  interlocking  communicates  with  individual  points  and  signals  by  means  of  a  duplicated  serial  data  link,   with  complex  data  coding,  which  interfaces  with  a  TFM  (Trackside  Functional  Module)  to  provide  parallel   input  and  outputs  at  an  appropriate  power  level  for  individual  signal  and  points.     The  required  functionality  at  a  particular  location  is  entered  as  configuration  data,  and  is  produced  in  a   bespoke  language  and  then  compiled.     Other  computer  based  interlockings  are  in  use,  including:     Interlockings  developed  in  mainland  Europe  and  customised  for  UK  application     Successor  interlockings  to  SSI  which  operate  in  a  very  similar  manner  but  on  a  more  modern   hardware  platform     6.5   AWS     Automatic  Warning  System  was  developed  in  the  1950’s  to   provide  the  driver  with  warning  of  a  signal  which  was  not  at   green.  It  consists  of  a  pair  of  magnets  (1  permanent  and  1   electro)  mounted  between  the  rails  about  200m  before  each   signal.       If  the  signal  is  green,  the  electro  magnet  is  energised,  and  the   driver  receives  a  bell  sound  in  the  cab.  In  all  other  cases,  the     Prepared  by  Neil  Porter   Page  6  of  8   Introduction  to  Railway  Signalling       v1.0_Additional  reading.docx       Introduction  to  Railway  Signalling                 permanent  magnet  causes  a  horn  to  sound  in  the  cab,  and  the  brakes  to  be  applied  if  the  driver  does  not   acknowledge  it  within  a  certain  time.     The  AWS  track  mounted  equipment  is  illustrated.     6.6   TPWS   Train  Protection  Warning  System  was  developed  and   installed  across  the  network  in  the  early  2000’s  in   response  to  a  number  of  high  profile  accidents,  notably   at  Ladbroke  Grove  in  1999.  It  is  designed  to  largely   mitigate  risks  associated  with  drivers  inadvertently   passing  signals  at  danger.     It  operates  by  means  of  pairs  of  antenna  (loops)   mounted  in  the  track,  which  when  the  signal  is  at   danger  are  energised  with  a  pair  of  frequencies  in  the   65kHz  range.       One  pair  of  loops  is  mounted  together  just  past  the   signal.  The  first  loop  frequency  arms  a  timing  circuit  on   the  passing  train,  and  if  within  a  preset  time  the  second   frequency  is  detected,  the  system  triggers  a  brake   application  and  the  train  is  stopped.  Thus  with  the  two   loops  together  a  train  passing  the  signal  is  stopped.  This  pair  of  loops  is  known  as  the  “Train  Stop”.     A  second  pair  of  loops  are  located  a  few  hundred  metres  on  the  approach  to  the  signal,  and  are  separated   by  a  pre  determined  distance.  A  train  passing  over  these  loops  above  a  preset  speed  (determined  by  their   separation)  will  also  be  triggered  and  bought  to  a  stand.  This  pair  of  loops  is  known  at  the  “Overspeed  Trap”   The  speed  setting  is  designed  to  be  above  that  which  a  train  should  be  travelling  to  safely  stop  at  the  signal.     The  photograph  shows  a  single  TPWS  loop.     6.7   Signalbox  control  and  display  systems     There  are  two  key  types  of  signalbox  display  systems,  ignoring  mechanical  signalling,  where  the  display   system  is  broadly  to  look  out  of  the  window!     6.7.1   Panels     These  consist  of  a  mimic  of  the  track  layout,  usually  etched  onto  small  metal  tiles.  The  signaller  can  control   the  signals  /  points  etc  by  means  of  individual  buttons  and  switches,  and  is  provided  with  information  on  the   state  of  the  system  by  means  of  illuminated  displays  within  the  panel.       There  are  many  panel  signalboxes  in  use  in  the  UK  covering  hundreds  of  route  miles  each.     6.7.2   VDU  based  displays     These  are  a  logical  extension  of  panels,  but  utilise  VDU  technology  to  provide  the  same  and  sometimes   enhanced  information.  The  signallers  control  interface  is  via  a  keyboard  and  mouse.  This  is  the  predominant   form  of  control  and  display  system  for  new  signalling  systems.       6.8   Ancillary  items     The  key  components  of  the  traditional  signalling  system  have  been  dealt  with  above;;  the  following  are  a   small  subset  of  the  remainder.   Prepared  by  Neil  Porter   Page  7  of  8   Introduction  to  Railway  Signalling       v1.0_Additional  reading.docx       Introduction  to  Railway  Signalling                   6.8.1   Location  Cases     These  are  lineside  steel  cases  containing  the  power  supplies  and   control  equipment  for  signals,  points,  axle  counters,  track  circuits  etc.   The  photograph  shows  a  location  case  under  test.     6.8.2   Train  describers     These  are  non  safety  critical  computer  systems  which  display  to  the   signalman  with  a  unique  alpha  numeric  description  of  each  individual   train  to  assist  the  signaller  with  routing  decisions.     They  obtain  the  train  location  and  movement  information  from  the   signalling  interlocking.       In  turn  the  train  describer  can  port  train  movement  data  onto  a  large  range  of  further  systems:       Automatic  route  setting  systems  to  take  away  some  of  the  signaller’s  workload  in  setting  individual   train  routes,  and  enable  signallers  to  supervise  more  train  movements     Passenger  information  systems  to  drive  station  displays  and  internet  updates  on  train  running     Train  performance  monitoring  systems       6.8.3   Level  Crossing  protection  equipment     A  more  specialised  signalling  sub  system  deals  with  the   management  and  control  of  conflicts  between  road  and  rail   vehicles  at  level  crossings.  Equipment  includes  road  lights,   barriers,  CCTV  monitoring  systems.     A  typical  installation  is  illustrated.     7.   Cab  Signalling       This  is  the  next  generation  of  signalling  systems  which  seeks  to  abolish  lineside  signals  and  provide   comprehensive  displays  in  the  driver’s  cab  to  indicate  the  speed  he  may  travel  at  and  the  limit  of  his  authority   to  move.  They  ultimately  aim  to  also  replace  infrastructure  based  train  detection,  by  enabling  the  train  to   know  its  precise  position  and  report  it  to  the  signalling  system  via  a  secure  high  reliability  radio  link.     The  key  system  is  ERTMS  –  European  Rail  Traffic  Management  System.                       Prepared  by  Neil  Porter   Page  8  of  8   Introduction  to  Railway  Signalling       v1.0_Additional  reading.docx