Is Trump really a train wreck if he's consistently leading in national and state polls? Sure, his campaign is held up by what I can only assume is some form of dark magic or the most elaborate prank of all time, but it's held up nonetheless.
The reference to train wreck is to what Trump is doing to the GOP: the nominating process, the identity, the brand, etc. Not his campaign itself; he is the engineer on a joy ride that is going very well indeed, for him.What's going on with Trump and the GOP isn't a trainwreck, it's a rocket launch. One that leaves the launchpad on fire and possibly destroys it, and the rocket itself may still explode, too. But the burning launch facilities are the result of years of mixed neglect and carefully placed explosives.
The Clinton campaign does look like a slow-motion trainwreck, to me. Hillary's unwillingness to take a stand on anything is becoming sadly comic. Many metrics that they'll need in the general are dropping. They're losing some support are more enthusiasm to Sanders, and can't seem to even compete with him. The scandals are petty (so far) but keep piling up, like gritty sand. What was presumed to be a coronation now comes across as a dull, uncertain crawl.