gonna be interesting what ends up happening when the UK starts inevitably running out of TOPS locomotive classes
for context, in the UK, a multiple-unit's class number is a 3-digit number, and locomotives get a 2 digit class number, meaning that locomotives are inherently far more limited
the TOPS class numbers for locomotives that are no longer in active service can in theory be overwritten, but I doubt it'd be a simple thing for them to do, and there are some holes in the allocated classes but the numbers do hold at least a small amount of information, with the first digit of a diesel locomotive's number being roughly what the locomotive's type rating would've been back in BR days (which depends on horsepower)
this is very rough though, ie class 20s are type 1, and yet are class 20
so.. not sure what will happen
will we get TOP class 901 for a locomotive lmao