I thought it would be fun to share some pictures of our train themed aquarium, in case there are any other combination miniferroequinologist-aquarists here on the forum. We have a 55 gallon tank, complete with a sunken G-scale mogul. The locomotive has an air tube run up through the boiler so bubbles stream out the smokestack:
The engine itself is a cheapo plastic one that we picked-up at a flea market for $4. My daughter and I stripped out all the metal parts (they're bad for the fish) and stickers, threaded a plastic tube and air stone out to the smokestack, added fishing weights to ensure it would sink, reassembled the train using 100% silicone, and installed in the tank! It worked out pretty well that the engine was already slightly melted when we bought it... gives it more of a realistic/banged-up look.
It's always a big hit, especially with kids, whenever we have company over.
-Dustin