Some mods like CIT or item swapper (mostly clientside mod) allow to load and deploy custom content: like new models without overwriting any existing resources, or block swapper palettes trough resourcepacks.
What if that can create new cosmetic (or even structural) entries to conductor skins, bogies, tracks, maybe even sounds and condition they played at.
Using that stuff on a server is probably other can of worms, I think it's gonna bound to ehatever resourcepack is on a server.
And just to clarify, I'm not talking about just changung textures or models that resourcepacks do already by default.