Coaches usually have a 4-, 5-, or 6-digit number, where the first two digits denote the year of construction (e.g., 8439 denoting a coach built in 1984, or 92132 denoting a coach built in 1992). In some cases the first two digits may represent the year the coach was transfered to the zonal railway, and sometimes the year represented is the year the coach was rebuilt. One exception are some of the Rajdhani rakes of Northern Railway, which have coaches numbered 1XXXX (15XXX). (Not all NR Rajdhanis have such coach numbers; 2951/2, 2953/4 don't.)
An alphabetic suffix may also appear (see below). Many older coaches which had 3-, 4-, or 5-digit serial numbers are being renumbered to conform to this scheme. Often...
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the zonal abbreviation is prefixed to the number, so that a coach may be ‘ER 89472 A’, or ‘SE 978052 A’ for instance.
From 2000 onwards, the year of manufacture is indicated ‘00’, ‘01’, etc., as expected, in the initial digits, e.g., ‘SE 018051 A’. Occasionally, some combination zonal prefixes are seen, e.g., ‘SK 01252 AB’ (seen on a WGSCN coach of the Hazrat Nizamuddin - Vasco Goa Express [6/03]), where the ‘SK’ indicates a coach jointly belonging to / maintained by South Central Railway and Konkan Railway.
On SER, many coaches have 6-digit numbers (e.g., 898439/A) where an ‘8’ has been inserted as the third digit into a 5-digit number in the above scheme. ‘8’ is the zonal number of SER in the train numbering system. For some time (1998-99), ER and NFR also followed this pattern, adding a ‘3’ or ‘5’ as the third digit, respectively. Recently [3/05] it's been seen that some coaches with 5-digit numbers, e.g., on WR, have been renumbered with an extra '0' at the end, e.g., 00452AB is now renumbered as 004520AB.
Following the first block of digits described above, the next 2 or 3 digits form a serially allotted number within ranges that usually indicate the type of coach, as shown below. (Recent coaches all have 3 digits for this (a 5 digit number on the whole), using a leading 0 for the 1-99 range.) The serial number is allotted chronologically in the order in which the coach is received by the zonal railway, within the range for the coach type.
001-025 : AC first class. On NER, some MG FC coaches from 2000/2001.
026-050 : Composite 1AC + AC-2T
051-100 : AC-2T
101-150 : AC-3T
151-200 : CC (AC Chair Car)
201-400 : SL (2nd class sleeper)
401-600 : GS (General 2nd class)
601-700 : 2S (2nd class sitting / Jan Shatabdi chair cars)
701-800 : SLR
801+ : Pantry car, VPU, RMS mail coach, generator car, etc.
So, for instance, a coach with number 92172 is the twenty-second AC Chair Car coach received by the zonal railway in 1992.
If there are more coaches of a particular type than numbers available in the allotted range as described above, the excess coaches are allotted numbers in the high 800's, usually 875 and above. For instance, sleeper coaches have been spotted marked SR 96886A, and AC-3T coaches spotted marked SC 97906A. The ranges are also sometimes redistributed.
In 1999, ER was to get a lot of AC-3T coaches for Rajdhani rakes and the new Sealdah Shatabdi. Hence, its only AC Chair Car of that year was renumbered ER 99181A, keeping 30 numbers between 151 and 180 free for AC-3T coaches (in the event, it turned out that these were not used after all).