This tunnel was much praised and valued when it was built but as with much other 19th century engineering it is very flawed. The constant flow of sea water into it has been dealt with but only at the cost of high energy demands.
This route is being electrified but electrifying this tunnel is a modern problem. I would be interested to know how they are doing that, to avoid the overhead being affected by sea water. The tunnel is located on the main highish speed route between London and Swansea. HST diesel trains are the main users of this route, travelling between London and Swansea. There are also diesel DMUs from Bristol to Cardiff and Swansea. After electrification these would be replaced by electric equivalents. Probably some Cross-Country trains, from Cardiff to Birmingham would continue to be diesel.
The best solution would be to build a modern tunnel, using the knowledge accumulated in the more than a century since this flawed tunnel was built. Another would be to build a barrage to harness tidal power for electricity, and lead a new railway line across it.
Its main purpose was in taking the numerous coal trains from south Wales. As this industry is now defunct a lot of its usefulness has gone, so a new Severn rail crossing is not very likely. There is already a motorway road crossing which takes a lot of the passenger demand for travel.
I have been through it several times. The actual passage through it is as normal as for any other tunnel. For the passenger there is no hint of the difficulties in building and maintaining it.