You can also use On30 stock on HO rails. For example, the common Bachmann O Scale (NOT 'gauge'....as bnsf971 says. Gauge is strictly the distance between the two rails you're using...) Christmas train sets with their proprietary EZ-Track, which is HO gauge. The rolling stock is O size, or scale, and easy to see and handle (for older eyes), but it will run on your HO layout if the side clearances (technically called 'loading gauge') is sufficient to let the larger rolling stock move past safely. Think tunnel portals, bridge structures, building corners close to the tracks, trees, etc.