The G&O garden railroad ran TMCC outside for years without problems. We switched to Legacy several years ago.
The key to grounding both systems was to drive a grounding rod into the ground and connect it to the common on the transformer. Our Z-4000 is in the a shed that is about 6 feet from the nearest track.
Another key is to make sure that both your engine wheels and the track is clean. We always run a track cleaning car before running trains. Outside the track may look clean but it usually is not. I have had TMCC engines that would not run until I cleaned their wheels even though other TMCC and conventional engines did run on the same track.
NH Joe
