The lights went out. A great smoke went up. It shaped itself like a mountain seen in the distance, and began to glow at the summit. It spouted green and scarlet flames. Out flew a red-golden dragon – not life-size, but terribly life-like: fire came from his jaws, his eyes glared down; there was a roar, and he whizzed three times over the heads of the crowd. They all ducked, and many fell flat on their faces. The dragon passed like an express train, turned a somersault, and burst over Bywater with a deafening explosion.
(FR 1.i.27-28, emphasis mine)
'Like an express train' is of course a simile entirely unsuited to the pre-industrial world of Middle-earth. Many have called it an anachronism, and it is, broadly speaking, but, as Corey Olsen has noted more than once in my hearing, strictly speaking it is not, because the 'translator' of the Red Book has introduced this phrase, not the narrator. Presumably the narrator (Frodo) used a phrase or idiom that conveyed the same meaning, only with different words. The translator, however, wasn't sufficiently alive to the words he was using to realize the paradox he was creating.
Consider one of Aubrey de Selincourt's least happy translations of Livy's Latin:
The tribune would have been roughly handled but for the universal and determined support of the mob and the rapid filling of the Forum by excited men who ran from every part of the city to swell the crowd. Appius stuck to his guns, ugly though the situation was....
(Livy, Book 2, Chapter 56; emphasis added)
The events described here took place, according to Livy, in the year we would call 471 B.C. So clearly Appius, one of the consuls of that year, had no guns to stick to. The Latin for 'Appius...was' is 'sustinebat tamen Appius pertinacia tantam tempestatem,' which may be easily rendered into the following English: 'Nonetheless through tenacity Appius withstood so great a storm.' De Selincourt, however, in his search for a forceful metaphor lost sight of the literal meaning of the words he chose.
Thus we can understand 'express train' as precisely analogous to 'stuck to his guns', as an artifact of a translation that momentarily out of touch with the larger context of the words being translated.* And since Tolkien himself is the only 'translator' of the Red Book who lived in the age of express trains, he is poking fun at himself by not removing the 'anachronism', perhaps at first as unwittingly as de Selincourt later did with Livy.
Livy, 'The Early History of Rome: Books I-V of 'The History of Rome from Its Foundations,' translated by Aubrey de Selincourt, Penguin (1960, reprinted with additional material 2002).