Hello Casey, there are four software programs available, 3d Max (very expensive), 3d Canvas Pro, Train Sim Modeller (TSM) and GMax (cut down version of 3d Max which uses plugins* to convert objects into protocols for various games and sims.
*Not all supplied, depends on the game or sim GMax is bundled with. [Flight Simulator X Deluxe has GMax bundled SDK, which will also work with the MSTS plugin.]
There are certain modelling protocols that must be followed in order to have the object converted to work in the sim.
Hope this doesnt put you off.
Cheers Bazza