Zombies spawned on rails when summoned by got hurt zombies 

but there is unable to spawn mobs on rails

1. you can create a platform, and cover it with any kind of rails 

2. then put some zombies like in the picture.

3. use a sword sweep these zombies.

4. there will be some zombies spawn on rails, sometime, they spawn without checking light, also they'll try to attack creative mode player as survival players.