He knows it was somebody else on the train and he's dug up a witness he thinks can prove it.
5 seconds sound clip from the Double Indemnity movie soundboard.
You can hear this line at 01:24:46 in the DVD version of the movie.
Quote context
[...]
- Keyes is rejecting your claim.
- He's figured out how it was worked.
- He knows it was somebody else on the train and he's dug up a witness he thinks can prove it.
- Prove it how? If he rejects that claim, I have to sue.
- What about me and the first Mrs. Dietrichson?
- And about that black hat you were trying on before you needed a black hat.
[...]