If you're invoking Clarksonian sentiments about the wit of man, remember that any time he has asked 'how hard can it be?' the answer has almost always turned out to be 'extremely'.
For reference, the railway runs under the river Frome at Chipping Sodbury. This weekend multiple rivers burst their banks by a considerable amount around the Bristol area (I know from personal experience as I had to drive from Bristol to Frome (different river Frome, just for fun) and back on Sunday). As the railway is the lowest point in that area, it naturally collects the water. NR has built ponds and installed pumps, but the narrow channel of the River means they can only pump so much or they'll flood the river downstream. You've also got a really steep escarpment immediately east that Chipping Sodbury tunnel goes through, just to make it more awkward.
A 20km aqueduct to the Severn is theoretically possible, but would involve a vast amount of land take and engineering to deal with the local topography (the area is anything but flat until you get to the coast).
Oh, and this time Chipping Sodbury is closed for a landslip.