Mobs in minecarts are able to move up and down tracks on their own accord. This is the case for unpowered powered rails, unpowered activator rails, detector rails in both states and standard rails.

This is a fairly easily observed phenomena, but appears to made particularly evident with villages in minecarts pathfinding toward a village centre or animals pathfinding toward a player holding wheat.

The aforementioned bug has also been observed with villagers, pigs, horses, cows, sheep

In order to recreate this bug, load a number of mobs onto separate minecart tracks ~10 blocks long