Hi @Anonymous
At this time the Mobility Simulator on works with Design/Component Roads (depending on which version you are running). If you are actually using the InfraWorks Rail tool, this is only a sketch tool and will not work within the simulator.
That being said, an option would be to setup a road style to appear as a rail, and when you select your study area you will be prompted to convert the roads in order to run the simulator. This may not be ideal, but may provide an alternative until we get improved design tools for the rail features.
Also feel free to post this in the Ideas forum to highlight your need for such functionality.
Seth Hall
Product Owner
Model Builder