There have been a number of TOPS class numbers assigned to proposed locomotives that have not been built for one reason or another.
- Class 18 - A proposed new Type 1 locomotive proposed in the mid 1980s. No prototype model was ever produced.
- 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.
- 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.
- 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.
- Class 75 - Spare classification for projected "super electro-diesel" follow on from Class 73 and Class 74.