Railway Track Circuit Configuration
Double and Single Rail Track Circuit Configuration
Railway Track Circuit Configuration :– Double rail track circuit arrangements have both rails fitted with IRJs to completely isolate a track circuit. Impedance bonds are used when a traction current return path is required. IRJs are not required with jointless track circuits and fewer impedance bonds are needed. The double rail configuration is preferred for track circuit integrity.
Single rail track circuit arrangements have only one rail fitted with IRJs to separate the track circuits, known as the insulated rail. The other rail is electrically continuous and is known as the common rail.
Jointless Track Circuits
1. Insulated rail joints can be expensive both to install and to maintain, especially on tracks subjected to high speed, high axle weight traffic or where there is an intensive service.
2. The use of audio frequencies permits the physical limits of an individual track circuit to be defined by tuned short circuits between the rails rather than by insulation in the rails themselves. Consider two jointless track circuits abutting at a tuned zone as shown in Figure 1 Non-track mounted equipment has been omitted for clarity.
3. The tuned zone comprises a measured length of track with a tuning unit across the rails at each extremity. The track circuits operate at different frequencies and each tuning unit is designed to its own track frequency.
Switches and Crossings (S&C)
Double rail track circuits are not generally suitable for use through S&C.For a crossover, or more complex S&C, where it is not practicable to fit IRJs opposite each other in the six foot area of the turnout between the running lines. It is also not practicable, in electrified areas, to fit a set of impedance bonds at this point.
However, through simple S&C layouts (single turnouts) in non electrified areas, fully jointed double rail track circuits are permitted, provided that standard double rail IRJ and bonding arrangements can be applied.