[The Saucepan Man]

If one were to seek to explain the train references in a manner consistent with the 'Translator's Conceit', then one might speculate that this was a phrase inserted by the 'translator' of the Red Book of Westmarch in place of a phrase which would have no meaning to his audience, or alternatively that it was an example of 'translator's' licence.

I am not sure that this fully explains all the anachronistic references, however, particularly those included in The Hobbit and the early chapters of LotR set in The Shire. The Shire is portrayed in a manner redolent of English Edwardian society and is in many ways at odds with the medieval feel of the remainder of Middle-earth (or at least the Mannish domains). Hence, we have mantle clocks, umbrellas, pocket handkerchiefs, waistcoats and even footballs (the latter three, I suppose, not being entirely out of place in a medieval setting), and probably others that I have overlooked. It might also (and indeed has been) argued that tobacco, potatos and coffee are also misplaced in a Western Middle-earth supposedly modelled on Western Europe (although this could be explained in a number of ways). There is a thread around here devoted to anachronisms, and another devoted to coffee and other New World imports - I will see if I can root them out.

If, on the other hand, there were actually express trains existing within Third Age Middle-earth, I would look either to the Dwarves or the Dark Lord (perhaps developed from the flying mechanical war-engines that Morgoth empoyed in the assault on Gondolin).