Tons of stuff.
Start doing DLS searches for 36 inch and 3 foot gauge and variations on those plus searches for the various 3 foot gauge railroads such as D&RGW, RGS, and so on. Lots of great stuff on 3rd party sites as well.
Track, traincars, loco, bridges, buildings, and so on. You will be a happy downloader for some time.
Ben