How can I make an entity ride a minecart?

Hi,

I'm working on a railway mod and I cannot figure out how I can spawn a minecart and a custom entity inside as a passenger.

I'm using a spawn egg for the entity, so that once it spawns from the spawn egg, it summons a minecart and the train entity rides the minecart.

However, when it comes to spawning the minecart and making the train ride the minecart, I have no knowledge of making it happen. Could someone explain and if possible, give me screenshots?

Thanks.