Describe the suggestion
JSON tag for diesel locomotives so that diesels can only use fuels specified in the JSON, as opposed to the main list in the config. (If the tag isn't present the loco would just use the main list)
For example, it the JSON it might state "allowed_fuels": gasoline, and the locomotive would only be allowed to use gasoline.

Applications of the suggestion
This allows locomotives to use only certain fuels, without said fuel being available to all locos. For instance, an experimental hydrogen locomotive without allowing all diesels in game to be able to use hydrogen as a fuel.

I had also made a similar suggestion which apparently is "ready to implement": #1382 - your implementation looks easier to develop whilst mine looks more user-friendly for those with lots of packs by different authors

a better idea I've thought of would be to still specify the fuels in the main list, and just add a whitelist or blacklist json tag so that you can specify efficiency in the list not for each loco

also I would suggest having the ability to specify efficiency in the loco list but if you leave the efficiency blank in the loco list then it uses the main list - that way you can make a gasoline engine that can take kerosene at low efficiency and a kerosene engine that takes kerosene at high efficiency (and possibly not have kerosene on the main list at all)