OV: IMO McGrath's line can be made to scan (well enough for folk music) if "when" & "the" are both unstressed, so that one has a dactyl in place of a trochee -- adding a little rhythmic interest, in the spirit of English prosody. Furthermore, that version has logic on its side, for at the instant when the boiler burst, the driver was presumably in one piece. Indeed, it is conceivable that over the last 51 years I myself have corrupted the line.