Intermediate Block Section:
Intermediate Block Sections are provided to increase track usage in areas with absolute block operations where the distances between successive stations are large, causing each block to be very long. The heart of the absolute block system is the idea of only permitting one train ever to be on a block at any time; however, if the block is very long, this restriction reduces the possible traffic on the route.
An intermediate block section is an additional block section which does not necessarily correspond to any station, and which is provided solely to decrease the block lengths. Such intermediate block sections are generally used if block lengths would otherwise be more than about 10-15 km. IR prefers block lengths to be on the order of 4 to 8 km, under normal circumstances.Normally an intermediate block section is guarded by a single stop signal (and perhaps a distant) which is similar to the Home signal for a station. Once a train has crossed the intermediate signal another train may be allowed into the preceding block, just as with the block sections for stations. An intermediate block is usually controlled by the signal box at the preceding station. E.g., between Dahanu and Gholvad on WR is an intermediate block section, which is controlled by Dahanu in the Down direction and by Ghovad in the Up direction.
Many intermediate sections have automatic signals using track circuitry to detect the presence of a train; however, manual operations are not uncommon. An ‘intermediate block post’ is a station (a class ‘C’ station) at the boundary of an intermediate block section. Normally only a stop signal and a permissive signal are used for entry to the intermediate block. An intermediate block signal is usually provided with a SPT ( Signal post telephone) for the driver of a train to talk to the station master or signalman of the controlling station in case the signal stays on for long, as the location of the signal may be well out of visible or audible range of the station, making it hard for the presence of the train to be noticed by the station.
If the signal is defective, the driver informs the station master of the station in the rear (controlling the intermediate block), who then obtains a Line Clear notification from the station master of the next station (with exchange of private numbers) and then authorizes the driver to proceed past the defective intermediate block signal (again with exchange of private numbers). If the phone itself is defective, the driver informs the guard and can then pass the intermediate block signal after stopping at it for 5 minutes, and proceeding at 15km/h (at 8km/h in low visibility) to the next station.
Once the next station is informed of the defective signal, the station in the rear is also informed of the situation and thereafter the entire stretch between the two stations is treated as a single block section. At that point following trains from the station in the rear will be allowed into the block section with written authorization to pass the defective signal. Intermediate block sections have no significance for crossing or precedence of trains (express/passenger, freight/passenger) as they are purely provided to decrease the headway or spacing between successive trains. Usually, there is no interlocking of any sort provided (i.e., there are no points to route a train differently as the intermediate block signals change, there are no sidings or loops at the signals).