Oh, he's just giving it a big buildup. Actually, it was simple enough.

3 seconds sound clip from the Stalag 17 movie soundboard.

You can hear this line at 00:53:59.077 in the Blu-ray version of the movie.

Quote context

[...]

- How many ships did you lose?

- Hmm. About half the group.

- Flying out of England?

- Yes. Waddington. 92nd bomber group.

- Hey, lieutenant, how'd you blow up that train with 50 guards around?

- Well, I, heh... Just lucky, I guess.

- Don't let him kid you. Cagney couldn't have pulled a sweeter job.

- Here's what happened. We're waiting at the depot at Frankfurt. Understand? When an ammunition train comes through the longest ammunition train you ever saw. Understand?

- Oh, he's just giving it a big buildup. Actually, it was simple enough.

- I just drifted into the men's room, fixed myself up a little time bomb, broke open the window and when the ammunition train started pulling out, I just tossed the thing into an open car. I guess there must have been some straw in there on the floor.

- Yeah. And about three minutes later you can hear it. Boom! Understand? Broke every window in Frankfurt. Understand? It was gorgeous.

- Wait a second. I'm not through. Understand?

- I wouldn't talk about things like that.

- They never caught on.

- They may. That's why I'd keep my mouth shut.

[...]