The length of track connected by an electric circuit to the signal cabin, block telegraph apparatus, etc. required for indication of light is called a track circuit. Thus, a track circuit functions to indicate whether the track is occupied or not.
Special precautions are necessary in the maintenance of track-circuited lengths as accidental short circuiting of a circuit in the track may cause serious delay in traffic. Some of the noteworthy points in this regard are as follows.
(a) The permanent way inspector should instruct the staff not to touch or place any tool or metal object across the two rails in the track, as this will cause short circuiting of the track circuit.
(b) All gauges, straight edges, and trolleys used in the track-circuited length should be insulated.
(c) The ballast must be clean throughout the track-circuited length and care should be taken to see that the ballast is kept clear of the rails and rail fastenings. The clearance from the foot of the rail must not be less than 40 mm.
(d) Proper drainage of the track should be ensured to avoid the flooding of the tracks during the rains. Ashpits, water columns, and platform lines should be specially watched.
Track-circuited sections are electrically insulated from the rest of the track by insulation joints that are present on either side of these sections. The most common type of insulation joints are as given below.
(a) One type of insulation joint is made up of a planed fish plate fitted with fibre or nylon insulation consisting of ferrules or bushes for the fish bolts, side channels between the fish plates and rails, and end posts between the rail ends.
(b) The other type of insulation joint consist of rubber-coated fish plates. There are two designs of this insulation joint. In one design, the fish plate is coated with synthetic rubber or nylon on the fishing surface. In the other design, the fish plates are completely coated with rubber or nylon, including the bolt holes. These insulation joints were tried in the field but proved unsuccessful owing to the fact that the rubber coating gets cut easily.
(c) Recently, insulated glued joints have been developed using resin adhesives. These joints consist of web-fitting type of fish plates glued to the rails using a high polymer adhesive and bolted with high tensile bolts. Insulation is provided by special insulating side channels, bushes, and posts made out of fibreglass cloth. These joints do not exhibit the movement and hence all maintenance problems are completely eliminated.