Now that would mean either killing him on the train or, and this is where it really gets fancy, you kill him somewhere else and put him on the tracks.
9 seconds sound clip from the Double Indemnity movie soundboard.
You can hear this line at 01:19:18 in the DVD version of the movie.
Quote context
[...]
- This Dietrichson business, it's murder and murders don't come any neater.
- Go ahead. I'm listening. (2)
- You know what? That guy Dietrichson was never on the train.
- Two possibilities, and I personally buy the second.
- You're way ahead of me, Keyes.
- Well, look, it was like this. They killed the guy, the wife and a somebody else...
- And the somebody else took the crutches and went on the train as Dietrichson...
- Then the somebody else jumped off...
[...]