As Mike said, as long as the water is below the rails, and the speed is low, the traction motors are fine. If the water is over the rails, the limit I once heard was two inches, but the advice that followed was "don't try it". The real problem is if the roadbed is washed out, and as long as the water is running under the rails, it's "probably" safe, as one can see low spots. Here you can see the water running over the top of the roadbed, so it's "probably" sound. Even so I wouldn't want to try it without orders from the dispatcher!
Steam can run through water as long as it's below the firebox, but it doesn't do the bearings any good, and there's a real danger of not being able to see a washout.
