Train Physics Problem

Hi,

i’am using the following two assets in my project:
Old Railroads Tracks: https://www.assetstore.unity3d.com/en/#!/content/35445
low poly TGM4 with wagons: https://www.assetstore.unity3d.com/en/#!/content/33105

I use for the rail tracks mesh colliders.
The front and the last axis from the locomotive has two wheel colliders each, the other two axis have not colliders. The root object from the locomotive has a rigdbody with the mass 1500.

On straight rail track parts the locomotive drives without problems on the rail tracks, without leaving them.

But on curved rail track parts the locomotive jumps from the rail track.

How can i solve this problem ?

I’am using the simplecarcontroller script from this tutorial for the locomotive: