Remember the movie, Unstoppable?

A massive, highly toxic locomotive roars out of control, threatening to cause devastating havoc.

To start with, nobody takes the threat seriously; it's deemed a 'coaster' that can be easily stopped.

But by the time the rescue team catches up with it, the train's running on full power and has accelerated to maximum speed.

At this point, the horrified experts consider deliberate derailment.

The result?

The train, by now a steaming, hooting, hissing beast, simply charges through the attempted derailment.

I won't spoil the ending for those who haven't watched it.

But I can't think of a perfect metaphor for this presidential race, in which Donald Trump last night all but sealed the Republican nomination.

The Trump Train has so far run over 17 Republican opponents and swatted away a derisory effort by GOP grandees to derail it.