The development of signalling systems pursued transitions; from a mechanical to an electro-magnetic-relay system, and finally to a microelectronics system.
In order to materialize high-speed and high-frequency operation, semaphore signals and mechanical interlocking systems had to be replaced by color-light signals and relay-interlocking systems with track circuits and electric switch machines.
The relay-interlocking system contributed to the safety of train operation. It was employed successfully for more than 30 years until the first electronic interlocking system was introduced in 1985. During this time, there were many efforts to improve its reliability, maintenance, and other features by solving some of its problems. Some examples of related on site work are as follows.
In the early stages, the wiring of relays on the back of relay racks relied on a manual soldering method. This was soon replaced by Faston terminals with utilizing a compressing tool, which facilitates the work. The result, however, produced another problem; as the wiring becomes complex, the Faston terminals were often disconnected by slight shocks from outside, for which an anti-disconnection attachment had to be provided.
As for the material used for relay bases and relay receptacles, phenol resin utilized in the early stages was unsuitable in its tendency for "metal migration", a slight current leakage through the wet dust between the terminals causing the plate metal of the terminals to migrate and finally leading to a short-circuit. The temporary countermeasure for this is cleaning between the terminals, and the permanent one, replacement with anti-migration type resin.
Of course, the problems described above and others were soon solved when standards were set, although on site repairs and replacement took considerable time.
The relay-interlocking system provides many functions, however, it requires a large number of relays, substantial space for equipment and an adequate power supply. Furthermore, as relay failure also increases in proportion to the number of relays, recording devices which record the movements of relays were required for trouble-shooting in large stations. It seems likely, therefore, that the time has come for the relay-interlocking system to relinquish its role to the next generation system.
The first electronic-interlocking system, SMILE (Safe Multiprocessor system for InterLocking Equipment), was developed and put into operation in 1985 at Higashi Kanagawa station. This system has various automated functions such as automatic route
