mobs jumping on minetracks

Well, I was making an update video and while I was by the Activator Rail, a creeper came over. He walked right onto the rail (which I thought was made impossible in the new update) and started jumping on it. He looked down at the rail and just began hopping in the air. If it makes a difference, the Activator Rail was lit. After jumping for a few seconds, he stood still, then walked off.