JB BAHN is a program for the simulation of train systems. It's predecessor was made for streetcar nets only, but today you can built railway lines, too.
If you are interested in Model Railroading, but you don't have a Layout, then BAHN can help. Even of you do have a Layout, it is often useful to model changes before you make them.
With this program several hundred kilometers of track and over 1000 trains (simultaneous running) can be modeled on the PC, even in a laptop.