It should be possible to designate certain parts to be usable on more than one locomotive by placing that part into a common category, sort of like an OreDict tag.

For example, the F7, SD7, and GP7 all use the same exact prime movers, traction generators, and traction motors. The prime mover parts for each could be categorized as "16-567B". As a result, prime mover components built for a GP7 should also be able to be used to build/repair a SD7 or F7. To balance crafting costs, the resource costs for each part type in a single category could be averaged between each other or given a specific resource cost.

The F7 and GP7 also share the same "Blomberg B" trucks, so trucks/bogies crafted for one should also be usable on the other if the trucks/bogies for each are both configured to be in the same category.

This would better represent the benefits of standardized locomotive design, both for historical/realism value and for players' in-game convenience.

One more point to add - a part taken from one piece of rolling stock to build a different piece of rolling stock can adopt the part model used by the target piece of rolling stock when placed, so no mix-and-match models would be necessary.

That makes it a bit more possible to implement. Mixing models is not something IR is currently built for. How do you imagine specifying ingredient cross compatibility as a modeler?

I think the part category could be specified in the .json, with a line listing the part name as spelled in the modeling software and one or more categories applied. If the part is not listed then it will not be placed into any category.

It is possible that a part category will not have identical spelling when used by different modelers (ex. "16-567B", "16 567B", "16 567-B" so I think multiple categories should be applicable to single part to cover these possibilities. This could also be used for resource packs designed for simplistic gameplay where otherwise non-identical parts are still designed to be interchangeable (e.g. placing all piston parts in a pack in a single generic "piston" category"