I think that's a little off. He found the cables, but was nowhere near the plunger. He was about to break the cables to save the bridge, when he said "What am I doing?" or to that effect, and as he hesitated he was shot, as ordered by the spy guy, what's his name.
I'm sure of it!
( Ah yes, Jack Hawkins! )
... also, it's THE BRIDGE ON THE RIVER KWAI
... ah yes, I remember it well!
"Recognizing the dying Shears, Nicholson exclaims, "What have I done?" Warden fires his mortar, mortally wounding Nicholson. The dazed colonel stumbles towards the detonator and collapses on the plunger, just in time to blow up the bridge and send the train hurtling into the river below."
I guess I'd better watch it again!
Actually, Colonel Nicholson fell on the plunger