Not that deep. As a rule*, as soon as the tracks are submerged the operating speed goes down... way down to 5mph or less. Once the water rises three inches above the top of the rail, it's goodnight, Irene.
*As a rule meaning individual railroads may apply more restrictions for their territories various conditions and locations.
Added... Oops, didn't see Andy had replied... I yield to the Gentleman from Omaha...
I've never been dead in the water, but I have been held up by storm caused fallen trees a couple of times.
Former Amtrak Conductor