try this for some off-the-wall thinking -
an electric motor mounted on the upper deck driving bevel gears to a diveshaft running down one of the legs to other bevel gears that drive a driveshaft mounted tranversely across the tracks to the adjacent leg with the wheels being attached to this driveshaft. Repeat at at the other end of the deck to power all wheels.
OR - extend the drive from the motor to the other end and use the same "down the leg" drive arrangement
Unanderra in oz
Any of our ''open frame''motors will operate submerged so long as the waterdoesn't have enough chemical in solution to become a conductor. The limiting factor is bearing lubrication, The speed of the armature causes a friction which the motor must overcome, but this appears to be a non-factor. Try it with a junk (but operational) motor, in tap water if you don't believe. I have even done it with 110 volt motor submerged in water full of iron grinding dust to prove the point. The motors may rust to the point of failure, but they will run until then.
Another gem of useless information from the
____________________ Fix it again, Mr Gates--it still works!"
Actually the drive suggested by oztrains is probably the way to go. Delrin bearings on NS axles for the wheels. So it is doable. And will work. But the water will be the unreal item to deal with.
____________________ Junk is something you throw away three weeks before you need it.