Я полагаю, что "железнодорожный туннель" - это вольности перевода, причем не качественного. У профессора было так:
"In a hole in the ground there lived a hobbit. Not a nasty, dirty, wet hole, filled with the ends of worms and an oozy smell, nor yet a dry, bare, sandy hole with nothing in it to sit down on or to eat: it was a hobbit-hole, and that means comfort."
Как видите, ничего подобного.
It had a perfectly round door like a porthole, painted green, with a shiny yellow brass knob in the exact middle. The door opened on to a
tube-shaped hall like a tunnel: a very comfortable tunnel without smoke, with panelled walls, and floors tiled and carpeted, provided with polished chairs, and lots and lots of pegs for hats and coats - the hobbit was fond of visitors.Конечно, tube имеет одним из своих значений подземку в Лондоне. Но основное-то значение это труба, туннель