As said in the in-game guide and on curseforge page, train cars should be able to load mobs. I tried with villagers, bees, sheeps but none of them can load in the car. Im using the latest 1.18.2 version

It's possible but a bit hard. Try dropping the mob on top directly. I will keep this issue open since I've noticed it's substantially harder than with minecarts for some reason.

In the future we want to have some way to load/unload mobs with docks