At the end of the movie the team floats down the river at night and sets the explosive charges at the base of the piers for the bridge. The following morning the charges are exposed because the river level has gone down during the night.

Now watch the big climax of the movie. Alec Guinness falls onto the plunger and the charges explode just as the train is crossing the bridge. But notice that the charges are now up at the level of the bridge deck and not down at the bottom where they were originally placed.