The British Transport Commission, later British Railways, used engine power output to categorise its new main line diesel locomotive fleet following the 1955 modernisation plan. These categories were from Type 1 to Type 5 and are listed below.
Type 1
changeUp to 1,000 bhp
Type 2
changeFrom 1,001 bhp to 1,499 bhp
Type 3
changeFrom 1,500 bhp to 1,999 bhp
Type 4
changeFrom 2,000 bhp to 2,999 bhp
Type 5
changeFrom 3,000 bhp