Railway signalling Solid State Interlocking (SSI)

Railway signalling Solid State Interlocking SSI

Indian Railway Signalling Metro Rail General Rules Network Rail

Railway signaling Solid State Interlocking SSI 

SSI Information Flow


Classification of SSI

  • SSI can be classified on the basis of

–Type of Redundancy used

  • Dual HW
  • Dual HW with 100% stdby
  • TMR
  • SW redundancy

–Whether Object Controllers are provided or not

–Type of Data Programming used – Free wire or Geographical

Object Controllers



Types of  Yard Data Programming

2 Ways of Yard Data Programming:

–Free Wire Programming &

–Geographical Programming


Free Wire Programming

  • Both – Interlocking Rules as well as Yard Specific data entered.
  • Cumbersome but Flexible



Architecture Geographical Programming

  • Interlocking rules embedded into the executive software
  • Only Selection table fed into the SSI
  • First time, time consuming
  • Later, adapting to any station – easy
  • Change of Interlocking rules or Practice – Executive SW to be changed


Various designs of SSI based on Different Architectures


Single Hardware Architecture

1 Microlok US & S
2 VPI (Vital Processor Interlocking) ALSTOM
4 VHLC GE Transportation (Formerly Harmon)


2/2 Systems

1 EBILOCK 850 Bombardier Transportation
2 EBILOCK 950 -do-
3 ESA 11 AZD Praha (Czeck)
4 ESA 12 -do-
5. SICAS S5 Siemens AG


2/3 TMR

1 SIMIS W Siemens AG
2 SSI Alstom
3 ESTW L90 Alcatel
4 SICAS Siemens AG


Features of Various SSI Systems

Microlok II

HW Single (Hot Stdby by using 2 Systems)
SW Diverse (Partial)
Object Ctrlr No
Validation HW – MIL Std, SW – 3rd Party by TUV to CENELEC Std.
Data pgming Free wire
Approx Cost 50 – 70 Lakhs

VPI (Alstom)

HW Single Processor
SW Diverse (Partial)
Object Ctrlr No
Validation Developed to ORE Std.Approved by London Underground
Data pgming Free wire
Working Since 1980s


ASCV (Alstom)

HW Single Processor (With Stdby Proc)
SW Diverse (Partial)
Object Ctrlr YES
Validation Details not available
Data pgming Free wire


EBILOCK 950 (Bombardier Sweden)

HW 2/2 – (100% Stdby for main and commn. Processors)
SW Diverse
Object Ctrlr YES (2/2, No Stdby)
Validation Validated to Cenelec 126, 128, 129
Data pgming Both Geographical & Free wire


ESA 11 (AZD Praha)

HW 2/2 (With Hot Stdby Processors)
SW Diverse
Capacity 10-12 Lines (3-4 Way Stns)
Object Ctrlr YES 2/2 Architecture
Validation To Cenelec Stds

SIMIS (Siemens)

HW 2/3 With common I/O
SW Common
Object Ctrlr YES
Validation Data not avl
Data pgming Geographical


SICAS (Siemens) (Siemens Comp Aided Sigg)

HW 2/3 With common I/O
SW Common
Object Ctrlr Data Not Available
Validation To Cenelec SIL 4
Data pgming Geographical
Capacity 3-4 Lines
Upto 8 systems can be connected to enhance the capacity
Installed Germany, China, Korea & Thailand


SSI (Alstom)

HW 3-Feb
SW Single
Capacity 6-7 Line (504 I/P, 504 O/p)
Direct Drive Yes (Signals-128, Points-64)
Object Ctrlr YES (2/2 HW)
Validation To Cenelec by BR
Data pgming Geographical
Installations More than 280

RDSO Design

HW 2/2 (100% Stdby Possible)
SW Common/Diverse
Object Ctrlr No
Validation Validation by RDSO
Data pgming Alstom – Geographical
CGL – Free Wire
Approx Cost Rs.25 Lakhs + Relays
Installations 4 (1st installation 1995)

RDSO Type SSI Architecture

  • Microprocessor Interfacing – General
  • SSI Architecture:

–CPU, Read, Comparator, Relay Driver, WD

  • See-saw Operation
  • Self Checks:

–Signature, Data Loop, Toggle, Read backs, Memory check – RAM & EPROM

  • Shut down through WD Timer



  • Common Interface Circuit
  • Read I/P
  • Comparator
  • Relay Driver
  • WD

1 thought on “Railway signalling Solid State Interlocking SSI

  1. Hello

    Government of Bangladesh has published International Tender notice for –

    Design, Supply, Installation, Testing and Commissioning of Computer based Interlocked Signaling System at 7 Stations including interlocking of selected level crossings and installation of optical fiber cable based telecommunication system for block operation and CTC interface work ( Turnkey Works ) in connection with construction of 3rd and4th Dual Gauge Track.

    If you are interested in participating in the Tender, please let us know to send further details to you. We can be your Local Agent for performing at field level.

    Thanks and best regards. Mowla Phone / Whatsapp 8801819213824

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