First off, it should be pointed out that like all other model locomotives (except live steam), Trainz locos are actually electric. Don't believe it? Issue your steamer or diesel a command that lowers both pantographs in-game and watch it stop dead. (g)
Ahem. But seriously, were you thinking more of something like C&O 500 (2-D+D-2) and N&W 2300 Jawn Henry which were Baldwin coal-fired steam turbine locos with bodies somewhat resembling a road switcher? 500 strongly resembles a very squared-off steamer (with tender blended in) atop Centipede running gear; 2300 doesn't resemble anything else, but imagine a Kraus-Maffei crossed with half a VGN EL-C and all corners left sharp. Either would be easy to model, given minimal scale drawings.
Other than that, a shrouded Hiawatha steamer would come close if you hid the drivers; make it cab-forward with a streamlined cab...