Train Gods CAN be interesting if used correctly. They can be used to make an area that would otherwise be safe/overpowered more dangerous; e.g. There's a tunnel that leads straight to a team's base, but every 1-2 minutes, a train thunders down it. If you're in the tunnel when the train approaches, you're screwed.
That would be a case when a train would be fun and okay. Most maps (my own included) use them as just randomized, moving death pits.
Another thing that could be interesting if executed correctly would be to have a map fought ON a train god, similar to ctf_convoy. Even if it's just a minor area, it seems like an interesting concept.
There was a KOTH map I played in a gameday a while back where the point was on a (slow) train god and would back and forth across the map. This was a really awesome concept and I liked it, especially since the losing team could set up a sentry in a spot where the train WILL get to and annihilate the surprised defenders once the train rolls through the area.
This has nothing to do with anything, but I'd love to see a 5 CP map set up like Steel, where the final CPs for both teams are always available for capture, and as the points progress the final cap gets easier for the winning team to capture. This could lead to an interesting scenario that encourages the winning team to worry about defense, as a ninja Scout could sneak past their front lines and cap the point. Of course, the cap would have to have a long cap time and be easily defensible at the start, but each captured CP should eliminate some good defenses (for example, having the cap start in the heart of the base and be covered by awesome Sentry sightlines, but once a CP is captured, the platform it's on rises up a level that has less and less defensive ability) and perhaps shorten the cap time. Once all the CPs except last have been captured, perhaps have the CP be like Granary's last, but before then have it MUCH more defensible.
I would love to do it, but between Snowblind, Nippon, and (to a lesser extent) Dockyard and Constellation, I have enough on my plate. D: