Why did the first attempt to blow up the Kwai bridge on camera fail?

Name: The Bridge on the River Kwai
Released: 1957
Cast: William Holden, Alec Guinness, Jack Hawkins

Question

Accepted Answer

A cameraman hadn't signalled ready. With the blast called off, the train rolled straight across the intact bridge and derailed on the far side. They hauled it back and blew the lot up soon after.

The Bridge on the River Kwai

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