On my survival world I have a minecart elevator in the neather(the one where you just click on each minecart to go up), each of these minecarts are on a ladder and whenever I go to the nether I see that a ghast or a magma cube is riding the ladder. I dont know whether they're spawning on the ladders or on the minecart. They definitely cant be coming from the top of the minecart elevator because then they would be riding the toppest minecart, but thats not the case, they keep riding the minecarts in the middle.