interlocking using Solid State devices.
Interlocking ensures that there is no conflicting movement of trains on tracks. For instance, if there is a train on certain section of track, it is imperative that the signal at the beginning of the section is red (or At danger, as it is called in Railways) to ensure that any other train doesn't enter in that section. this is done in many places manually using levers, but that is an old, antiquated technology and puts limitations on how many trains that area can operate. hence IR is modernizing its n/w by replacing those levers with Relays.
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even relays have their own issues, as its an electromechanical device and despite it being faster, it still takes decent fraction of a second to act+has lower reliability owing to moving parts. hence the next step is to replace those relays with microprocessors, which can take action in microseconds and have no moving parts. since microprocessors (or electronic devices in general) are called solid state devices, the interlocking system utilizing them is called Solid State Interlocking.