Mobs are unable to cross minecart rails

I was exploring an underground mineshaft, when I noticed zombies in the shaft ahead. The closest one was standing in between the fence supports and was looking at me, but was not moving. In front of him were several rails. I tried moving side to side, to see if he would follow, but he simply stood there. He only started moving after I destroyed the rail. Later on, in another section, two zombies and a creeper were blocked by the rail, and walked around it as if it were a fence blocking the way. Spiders were partially affected, though jumping on it they would pause and begin acting as if under full sunlight for a moment before attempting to attack again. Mineshaft was located in Swampland biome.