Wüstling zombies are bigger than your standard zombies and carry a big weapon. They will start their Charge attack once you shoot them. It's important to know that they will charge exactly in the direction you shot them from. Keep that in mind when you start his attack.
Once you hit wave 11, Wünstling zombies will show up regularly. Let all zombies spawn in and lead the train in the right position before you shoot at the Wüstling.
I found that the pilot light area outside the bunker and the bunker stairs are a good spot to keep the train under control and set up your Wüstling shot.