The Taggart Tunnel in Atlas Shrugged was 8 miles long.
The following "Poisonous air…" link provides some historical recollections about the 1 mile long (6026 feet) St. Clair River Tunnel.
Poisonous air made St. Clair River Tunnel a deadly place to work
Wow, if that Wikipedia link is correct, China is currently building about sixty new tunnels longer than 8 miles. Added to the eighty-five they already finished, that’s a lot of Taggart Tunnels.
How many of them are under a ‘Dagny-style’ construction management?
How many of them would surrender their spare locomotive engine to a civil servant’s insistence?
The dangers described, generated by a mile of tunnel, are likely to be raised by some exponential factor to the length. Improvements, brought about by minds dedicated to various ventures, have made vast improvements in many fields, ventilation, machines that can lay track as they bore and also fabricate reinforced walls, metallurgical improvements in rail and locomotive applications, . . . have helped in these and other ways to mitigate the risks involved.
Seikanin Japan is 14.5 miles. Daishimizu is 13.8; people died of carbon monoxde while building it but not while traveling in it. Simplonis 12.3.
The Simplon casualties were by and large due to accidents. Asphyxiation, not so much.
This rail calamity in Canada in 2013 made me remember right off the sequence of employee specifics leading to the fictional crash Rand had crafted.