Where letters are used in designations, Russia uses the Cyrillic alphabet but, in this text, the Romanised form is used throughout.
Prior to 1912, there were various different methods of numbering in use. In 1912, locomotive classes were identified by a capital letter, often followed by a subscripted or superscripted letter indicating some feature or modification, completed by a running number.
In 1925, a numeric system using a series number followed by a 2-digit running number was imposed. This scheme had only limited application because of the inconvenience of allocating a new series number for every 100 locomotives.
The system introduced in 1931 used class letters taken from the names of various political heroes, followed by digits representing the axle-loading and terminated by a running number. This system was not without risk when the political climate changed. For instance, the 2-8-4 passenger class introduced in 1932 was originally called the 'IS' class (for Iosif Stalin) but, in 1962 when Stalin became persona non grata the class was quietly re-named 'FDp' where the 'p' indicated the passenger version of the similar 2-10-2 freight class called 'FD' (commemorating Feliks Dzerzhinski).
During and after the second World War, additional variety was introduced because the class letters were sometimes alphanumeric. So, some classes were known just by a letter (for instance, the 'L' class) whereas the 'P36' class used the factory project number and some politically-inspired class designations were used. After the War, the letter 'T' represented a captured locomotive and this was combined with a letter indicating a similar Russian-built class (for instance, the captured German 'Kreigslok' locomotives became 'TE' class. Running numbers could be 3-digit or 4-digit.
Further changes were introduced with dieselisation. With multi-section locomotives, an initial digit indicated how many sections, followed by 'TE' for diesel electric transmission and 'TG' for diesel hydraulic types. A third letter 'M' or 'P' indicated shunting or passenger types, respectively. Imported locomotives were identified by a further prefix, for example the ChME3 came from Czechoslovakia. Individual sections of multi-section locomotives were coded A, B, V, G (the first four letters of the Cyrillic alphabet).
After WWII, large numbers of main-line electric locomotives were produced, coded with a letter representing the builder and figures representing either the number of axles or the axle loading. In 1963, this was replaced by the prefix 'VL' and a class number up to 39 (for d.c. types) or 40 and above for a.c. types. Imported locomotives used letters representing the country of origin, as adopted with diesel traction.
Diesel multiple units were coded 'D', followed by a letter indicating the the builder (often 'R' for Riga) and a class number. Electric multiple units were similarly coded, but starting 'E'.
In 1984, an all-numeric system of identification was introduced for all locomotives and rolling stock, to facilitate computerisation. All numbers were 8-digit, divided into two groups of four for legibility. The system was unpopular and various problems arose. The first digit indicated the type of vehicle (0 for passenger coaches, 1 for traction units, 2 to 9 for freight vehicles).
For traction units (code 1), the second digit indicated the basic type of traction (steam, diesel, etc.), the third digit the intended purpose (passenger, freight, shunting etc.) and the fourth digit the individual class of traction. The 5th, 6th and 7th digit provided a group of 1,000 individual running numbers.
In all cases, the final (8th) digit was a checksum, calculated from the previous seven digits.
This article on numbering systems is adapted from Appendix 1 of Reference 1 which gives more detailed information (up to 1994).
References:
Reference 1: 'Soviet Locomotive Types - The Union Legacy' by A J Heywood & I D C Button (Frank Stenvalls Forlag) ISBN 0-9525202-0-6.
