Because you want model the railway in two dimension, it is related to your studied section. I mean your section is parallel or perpendicular on loading. If your section is perpendicular to loading, the simulation is easy in PLAXIS2D. You just use the dynamic load option in the software. But if your section is parallel to loading, it's not easy and you should simplified it like the following paper.
Conference Paper 2D Numerical Modeling of Differential Settlement in Transiti...