Railway track and electrification is the rails, the concrete sleepers that hold them, and the overhead power supply that gives electric trains their power. A railway cannot run without it, and most of it is built once and then used for decades.
The track has two main parts. The rails are the steel bars that the wheels run on. The sleepers are the concrete or wooden blocks laid across the ground under the rails, which keep the rails at the right distance from each other. Crushed stone packed around the sleepers holds them in place and drains rain water. Together, these are called the permanent way. They must stay straight and level, because a small bend can derail a fast train.
Electrification means supplying power to trains from wires above the track. On Indian Railways, the overhead wire carries alternating current at 25,000 volts (25 kV). A train collects the power from the overhead wire through a metal arm on its roof. Large transformers and circuit breakers sit along the line. A circuit breaker is a switch that opens by itself when a fault such as a short circuit occurs. This protects the power supply, the wires and the trains from damage. Power transformers and switchgear are also used in the electricity grid, as described under power transformers and switchgear. The railway takes its power from the grid through substations that step the voltage down.
Take, for example, a short circuit on the overhead wire of a suburban line near Kolkata. The circuit breaker opens within a fraction of a second and cuts the power to that section. Trains in that section stop. Engineers find the fault, repair it and close the breaker again. Without the breaker, the fault would keep drawing current and could burn the wire.
Sensors check the condition of the track, and the findings go to railway condition monitoring. The track and power carry trains, which depend on signalling for permission to move. Railways buy track and power through government tenders, as described under railway technology.