There have been a number of TOPS class numbers assigned to proposed locomotives that have not been built for one reason or another.
Diesel locomotives
Type 1 locomotives
Class 18 - A proposed new Type 1 locomotive proposed in the mid 1980s. No prototype model was ever produced.
Type 3 locomotives
Class 38 - Projected classification for new generation of Type 3 freight locomotives in the 1980s. The Class 38 was dropped in favour of the Class 60.
Type 4 locomotives
Class 41 - The second use of Class 41 was for a proposed class of Class 60 locomotives with 2,500 hp engines in the early 1990s. Class 48 - The second use of Class 48 was for a new class of freight locomotive in the mid 1980s.
Type 5 locomotives
Class 51 - Projected classification for the proposed "Super Deltic" locomotive intended as a follow on from the Class 50 and Class 55. Class 62 - Projected classification for proposed Type 5 coal train locomotives in the early 1990s. Class 65 - Projected classification for proposed 4,000 hp freight working locomotive intended for liner trains.
Electro-diesel locomotives
Class 75 - Spare classification for projected "super electro-diesel" follow on from Class 73 and Class 74.
Electric locomotives
Class 88 - Proposed electric version of the Class 58, powered through 25 kV AC from overhead wire. Class 93 - Proposed electric locomotive for use on the West Coast Main Line as part of the InterCity 250 project.