Q. What are the different classes of stations?
IR's classification of stations is linked to their rules for block system working.
Class A: Such a station is one where the Line Clear indication for the block may not be given unless the line where the train is to be received is clear at least for up to the starter signal (or, in some cases, for at least 400m ahead of the home signal). These are stations where many trains normally run through without stopping at a high speed, hence the need for the...
more...

safety margin to prevent accidents in cases of trains overrunning signals. [4/00]
Class B: Such a station is one where the Line Clear indication may be given before the section of the line within the station has been cleared for reception of a train. Branch lines and routes with lower running speeds fall into this category.
Class C: This is a station (or block hut) marking an end of a block section, with light traffic or where no trains are booked to stop, such as an intermediate block post. (Sometimes these stations exist only in the form of a signal cabin that controls the approach to another station.) Permission to approach may not be given for a train unless the whole of the last preceding train has passed complete at least 400m beyond the home signal and is known to be continuing on its journey.
Class D: This is a station which does not form the boundary of a block section but which does form a stopping place for trains. Trains are stopped by various ad hoc arrangements prescribed in view of the local conditions -- the driver may simply know to stop the train there, or it may be flagged down on demand, etc. Also known as a non-block station or flag station.
Stations of classes A through C are known as block stations. The Class C stations are also sometimes known as block huts or block posts. Class D stations are known as non-block stations - they do not mark the ends of block sections. (Note that there also exist Intermediate Block Posts where block sections can be terminated without the presence of a station.)
Any station that is not classified under one of the classes A through D is known as a Special Station and must use specific working rules issued by the Commissioner of Rail Safety.
Sometimes stations are classified as Class A if there is a bridge or steep gradient just before the station in one or both directions, so that it is not safe for trains to stop there before entering station limits.