An X.25 packet data network for railroad signal supervisory control

Supervisory control for the remote operation of railroad signal systems traditionally relied upon point-to-point communications circuits. The example presented here describes an X.25 packet data network constructed for the remote control of several new railroad interlockings on the MTA Long Island Rail Road, USA. The X.25 network uses commercial off-the-shelf equipment to provide redundant communications with automatic rerouting. This supervisory control system employs a high level of path diversity for maximum reliability. The network topology is a mesh consisting of two interconnected rings. Communication between sites is via digital transmission at 56 kb/s. Each site has two redundant nodal processors connected to two redundant programmable logic controllers. This paper describes the process leading up to the final implementation and the service history to date. It also provides background information on communications networks and the OSI Reference Model, operating experience with existing signal supervisory systems on the LIRR, future implementation plans and an extensive glossary.