For locomotive power classes used by British Railways between 1948 and 1959, see LMS locomotive numbering and classification.
The British Transport Commission, later British Railways, used engine power output to categorise its requirements for the new main line diesel locomotive fleet following the 1955 modernisation plan. The locomotives built and put into service are listed below classified with the TOPS class numbers that were introduced in the early 1970s.
Type 1
Locomotives classed as Type 1 were of 1,000 bhp or below.
- Class 14
- Class 15
- Class 16
- Class 17
- Class 20
- Certain members of Class 21
- Certain members of Class 22
Type 2
Locomotives classed as Type 2 produced between 1,001 bhp and 1,499 bhp.
- Certain members of Class 21
- Certain members of Class 22
- Class 23
- Class 24
- Class 25
- Class 26
- Class 27
- Class 28
- Class 29
- Class 30/31
Type 3
Locomotives classed as Type 3 produced between 1,500 bhp and 1,999 bhp.
Type 4
Locomotives classed as Type 4 produced between 2,000 bhp and 2,999 bhp.
- Class 40
- Class 41
- Class 42
- Class 43 (Warship)
- Class 43 (HST)
- Class 44
- Class 45
- Class 46
- Class 47
- Class 48
- Class 50
- Class 52
- Class 53
- Class 57
- D0260 Lion (prototype)
- DP2 (prototype)
Type 5
Locomotives classed as Type 5 produced 3,000 bhp or more.