The Severn Railway Tunnel
Great Western Railway, The Severn Railway Tunnel: Chapter 1 - A Tunnel or a Bridge
http://www.greatwestern.org.uk/severn1.htm

The Thames Tunnel
Short illustrated history of the first tunnel to be built under a navigable river and the first to use a tunnel shield.
http://www.brunelenginehouse.org.uk/tunnel.asp

The Mount Cenis Railway and Tunnel
This site is an anthology of 19th and early 20th century railroading. It is directed towards the enthusiast and civil engineer as well.
http://www.catskillarchive.com/rrextra/mrcenis.Html

Lovat Inc.
Custom design and manufacture of tunnel boring machines (TBM) utilized in the construction of metro, railway, road, sewer, water main, penstock, mine access and telecable tunnels.
http://www.lovat.com/

AlpTransit Gotthard AG, Luzern (Switzerland)
Constructing the longest railway tunnel in the world, through the base of the Alps between Switzerland and Italy. Information and history of the current and new Gotthard tunnels.
http://www.alptransit.ch/

Australian Railway Industry Corporation
Australian railway export association comprising companies supplying technologies to railway operations worldwide.
http://www.aric.net.au/

Railway Preservation Resources
Helping professionals, museums and private owners understand, conserve, restore and operate historic railway equipment. Historic railway consulting, research, conservation, restoration and collection management.
http://www.railwaypreservation.com

Zebedee Secure Tunnel
Establishes an encrypted, compressed tunnel for TCP/IP or UDP traffic between two systems. (C) [Unix, Windows 95/98/NT]
http://www.winton.org.uk/zebedee/

Bible Places: Hezekiah's Tunnel
Pictures and text illuminating the biblical site of Hezekiah's Tunnel
http://www.bibleplaces.com/heztunnel.htm

The Sutro Tunnel
Description of the Sutro Tunnel on Nevada's Comstock Lode, taken from Sam P. Davis' History of Nevada (1912)
http://www.nevadaobserver.com/The%20Sutro%20Tunnel%20(1912).htm