Investigation of Common Section Interlocking System for Addis Ababa Light Rail Transit

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Addis Ababa University


An interlocking is "an arrangement of signals and signal appliances so interconnected that their movements must succeed each other in proper sequence". Railway interlocking is a safety, monetary and environmentally critical system because its failure may cause serious consequences such as loss of human life, severe injuries, and large scale of environmental damages or considerable economic penalties. The safety and complexity of this system requires formal modeling and step by step refinement for its construction and development. Computer-based interlocking (CBI) is a device controlling the signaling equipment by computerizing both the electrical and mechanical systems of the existing interlocking device. CBI is preferred in Addis Ababa Light Rail Transit (AALRT) because of its simple lightweight structure, failsafe configuration, high-speed processing of data and simplicity in changing hardware and software. This paper dealt with an investigation of common sections interlocking system which ensured system a synchronization and concurrency by modeling suitable interlocking systems with the help of Petri net. Petri nets are a graphical and mathematical modeling tool applicable to many systems. Petri net is also used in railway application like scheduling rail operation, in supervisory control approach for railway network, modeling train control system for level crossing, and in developing interlocking and signaling system with safety verifications. At first, the physical layout of five stations of AALRT are being drawn using AutoCAD with its full components, such as signals, switches, and track circuits. After that, a proper model of interlocking system is being modeled with the help of Petri net software and the model alleviates the problem which has been encountered in the interlocking system by allowing the interlocking to check the trains asynchronously without any collision.



CBI, Interlocking, Petri net, AALRT, Interlocking table