Abaqus Contact Formulation

Dear all,

In modeling wheel-rail-contact, I use the node to surface contact discretization approach. The wheel nodes are the slave nodes and the rail surface is the master surface. I just adopt the default tracking approach and the default strict constraint enforcement method. But, after the train moves some distance, it'll return back to the original position before reaching to the final destination. The master surface is sufficiently long and I think the amplitude function, boundary conditions etc are defined clearly. So, what's wrong with this? Does this have something related to surface smoothining or the direct contact constraint enforcement method? Please forward me your comments either for this specific case or more generally for a moving load. 

Thank you in advance,

Amdebrhan