Based on the TOPS numbering system, diesel locomotives are numbered from one through 69 with the highest number being the most powerful locomotive. They are divided into six types beginning with the first sub-type of 01 to 14, which are smaller locomotives with less than 800 horsepower. The five higher types include: Type 1 (numbers 15 to 20 with 800 to 1,000 hp), Type 2 (numbers 21 to 31 with 1,001 to 1,499 hp), Type 3 (numbers 33 to 37 with 1,500 to 1,999 hp), Type 4 (numbers 40 to 53 with 2,000 to 2,999 hp) and Type 5 (numbers 55 to 67 with higher than 3,000 hp). Class 57 is the only exception with 2,500 hp due to its extra hauling power that is equal to a Type 5.
Electric locomotives, based on the TOPS classification are numbered from 70 to 95. They are divided into two categories: DC that are direct current locomotives numbered from 70 to 77, and AC, which are locomotives numbered from 80 to 92 that utilize alternating currents or both AC and DC systems. Locomotives in this classification can also be electro-diesel trains that use an electrified third rail or an internal generator.
A diesel multiple unit trains, also known as rail cars, consist of multiple carriages powered by one or more on-board diesel engines. The TOPS numbering system divides them into two overall categories based on transmission and year. Mechanical or hydraulic transmission units numbered in three groups: 100 to 131 for units built between 1956 and 1963, 140 to 144 for units built between 1981 and 1988, and 150 to 180 for units built from 1984 and later.
The diesel units with electric transmission are numbered in four groups: 201 to 207 for units built from 1957 to 1962, 210 for the 1981 models, 220 and 221 for units built from 2000 to 2002, and 251 to 254 for units built from 1959 to 1985.
The electric multiple units are divided into six groups based on electrical source: 302 to 312 for overhead AC models, 313 to 390 for second-generation overhead models, 401 to 438 for third-rail models in the southern region, 442 to 466 for second-generation third-rail models in the southern region, 480 to 499 for third-rail special types in the southern region, and 501 to 508 for either third-rail or overhead models in non-southern regions.
Locomotive-hauled coaches based on the TOPS classification are numbered from one to 99,999 and divided into several common categories: Mark 1 (mail and parcel coaches), Mark 2 (dining cars, standard and first classes coaches), Mark 3 and 4 (sleeping cars, luggage coaches, as well as standard, first-class, and dining coaches).
