Minecarts do not seem to be affected by resource packs and behavior packs properly. If you specifiy in minecart.entity.json that it should, for example, use a different model or texture from a different path than textures/entity/minecart, it just doesn't work. Same goes for the model - you cannot modify the name of the model, but you can reuse the default minecart name and modify that, and the game will use the modified version instead. But naming it differently just does not work.
While this might not seem like that big of a deal, it makes it impossible to do any sort of customizing, such as different textures for different variants (as specified in a behavior pack).
This bug also affects minecarts with chests, tnt, hoppers and any others. For example, the chest minecart always uses the default minecart geometry and texture, and the game does not care whether or not you specified custom textures in chest_minecart.json.
As a side note, adding components like attacking to minecarts also does not seem to work (why would I add this? Normally, when minecarts are quite speedy and bump into an entity, it would cause quite some damage...).
I've included an mcaddon file for testing purposes. What it should do: replace minecarts with ridable modelled trains. It should speed the train up when driving over a rail sensor, making it twice as fast (this works in version 1.14.1), as well as some other smaller modifications. Minecarts with chests should have a different texture (not working since version 1.13, if I recall correctly). This addon is also available from this GitHub repo.