ST5087 : Old railway and pumping station, Sudbrook This enormous building housed six Cornish steam pumping engines to drain the Great Spring that is always threatening to inundate the Severn Tunnel. British Rail scrapped these in 1968 in a great act of corporate vandalism. Had they survived another 6-8 years this would probably have been unthinkable. I've been in the house and it's most impressive. I've been in the tunnel too - awesome.
The best book on the construction of the tunnel remains that by Thomas A Walker - the contractor.