Most model railroad track planning software is Windows-only. Empire Express 1.5 runs on Macs — including OS X — and is relatively simply to use, but limited. For one thing, it doesn’t appear to handle grades and separations. This is important if you’re hoping that the track on top will clear the track beneath it, or if you’re trying to keep your grades under control. (I’m doodling with ideas of geared locomotives and logging railroads, which means grades can go nuts, but still.)