Several versions ago this did not happen. You can tell this is a bug because Mojang specifically programmed mob AI to avoid walking on rails, but as of 1.8 they now spawn on top of the tracks. As a consequence of their AI, both friendly and hostile mobs will be stuck on the tracks once on there.

They directly avoid walking on rails, but will indirectly fall onto them in an attempt to get closer to you. This causes an unusually high number of mobs on the tracks when you're trying to ride them at night.