Reinforcement Zombies spawn on rails when summoned Zombies got hurt, sometimes they spawn without checking light levels

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 zombiesspawn on rails, sometime, they spawn without checking light, also they'll try to attack creative mode player as survival players.