Yesterday, I heard from someone who owns an actual copy of the book it was in, Susan Barnett.
She gives the full set of verses as:
Dwarfs live underground,
Dryads in trees,
There are trolls and goblins and imps and ogres,
Bogeymen and kelpies but much better than these
And very much nicer is such a rogue
As the railway boggart.
He doesn't clank chains
But has lots of fun
In railway trains.
He's no long ago fairy,
elf or gnome.
He is not a fire drake or a pixie.
Doesn't haunt the bedrooms in a stately old home,
There's only one place to see all his tricks,
He's a railway boggart
Who likes to take pains
To do all his tricks
In railway trains.
Ghosties and ghouls which go
Bump in the night,
Or the mischievous Cornish sprite called a piskie,
Little outer-space men come and give you a fright
And leprechauns sometimes spit in the whisky!
A railway boggart
From all this refrains
And only does tricks
In railway trains.
Looking at my rendition of this, I had the second-to-last line of the verse I'd remembered wrong: I had "But plays all his tricks" instead of "But has lots of fun". It seems I was taught the other verses (badly!) and conflated one of the lines from there into the first verse. I do hazily remember the piskie bit, come to think of it.
Hopefully, this will satisfy the steady stream of 50-something people who have emailed me on the subject. Thanks, Susan!