In the good old days, as the last millennium was coming to a close, the trains moved around without communications. Telegraph & Telephony progressively came into existence in India through Railways requirements. Earliest lines sanctioned in the direction of Rail routes were from Calcutta to Peshawar and Bombay area. Later, for longer distances, IR started using microwave communications (2GHz and 7GHz (7.125GHz and 7.425GHz) for administration, 8GHz, and 18GHz for control communications) with backup wireline telephony. The microwave links besides having more bandwidth than the older telephony cables also avoid the problem of cable theft. Most links have 120 channels and more recent ones (post-1987) have 960 channels. The four major metropolises are interconnected by a digital 34+2 Mbps microwave channel. In addition, spread-spectrum CDMA communication is in use between a few stations (Mumbai-Wadi on CR, Wadi-Secunderabad on SCR). Other major routes, not covered by these, have UHF TDMA links.
With the advent of IT, computers have made the job of traffic controllers a lot easier. Today, major stations’ computer networks are also connected via trackside cables. Control communications and control (the SCADA system) for electric traction substations is usually done through trackside metal cabling. Since about 2000, a major effort has been underway to provide optical fiber communication links between stations. So far, fiber-optic links have been provided along the routes among New Delhi, Ahmedabad, Mumbai, Pune, Bangalore, Chennai, Hyderabad, and Kolkata. Since about 1999, handheld radio sets (walkie-talkies) have been issued to most drivers, guards, and other staff on the move. These handsets usually have a fairly short range (a kilometer or so). VHF radio sets have been installed in the loco cabs for a few important trains such as the Grand Trunk Express, Tamil Nadu Express, and the Rajdhanis and Shatabdis, for communication between the loco and station controllers. Some systems like the Delhi Metro also use mobile radio systems for train communication; the radio system is integrated into the larger system of communication, which includes optical-fiber communication between stations, etc.
Signaling is one of the most important aspects of Railway communication. In the very early days of the Railways, there was no fixed signaling to inform the driver of the state of the line ahead. Trains were driven “on sight”. But several unpleasant incidents accentuated the need for an efficient signaling system. The earliest system involved the Time Interval technique. Here time intervals were imposed between trains mostly around 10 mins. But due to the frequent breakdown of trains in those days this technique resulted in rear-end collisions. This gave rise to the fixed signaling system wherein the track was divided into fixed sections and each section was protected by a fixed signaling.
This system is still being continued although changes have been brought about in the basic signaling methods. Earlier mechanical signals were used but today block signaling is through electric instruments.In the mid 19th century mechanical interlocking was used. The purpose was to prevent the route for a train from being set up and its protecting signal cleared if there was already another conflicting route setup. The most modern development in signal interlocking is SSI- a means of controlling the safety requirements at junctions using electronic circuits which replaced the relay systems supplied up to that time.
In Indian Railways, first trial installation of SSI was provided at Srirangam station in 1987. Nowadays Track Circuits are used wherein the current flow in the track circuit will be interrupted by the presence of wheels and a “stop” signal will be shown. A “proceed” signal will be displayed if the current flows.Today the Mumbai Suburban Section is providing with Auxiliary Warning System. It continuously monitors the speed and whenever a motorman passes a signal at “red”, applies emergency brakes to bring the train to a halt.