A wider rim should be slightly more stable than a narrow one. But, I don't think you have to go overboard.

So, just about any "29er" rim should work for you.

Sun Ringle makes what they call a "DoubleWide" rim.

Velocity also makes wider rims including the Atlas, Blunt, and Dually.

You're probably up on different wheel sizes. 36" will give you more distance per pedal stroke for a direct drive pedal. That would seem to be a benefit for doing long rail rides which should be relatively flat.

There is also a 32" wheel that I think showed up on some Kent bicycles, which would give you a bit of an intermediate size. Of course finding spares could be a problem, but you'd get two rims and tires if you bought a bike.