Let the train pass, then dim your lights twice.
5 seconds sound clip from the Double Indemnity movie soundboard.
You can hear this line at 00:52:54 in the DVD version of the movie.
Quote context
[...]
- No speeding. You don't want any cops stopping you with him in the back.
- Walter, we've been through all that so many times.
- When you leave the highway, turn off all your lights.
- I'll be back on the observation platform. I'll drop off as close to the spot as I can.
- Let the train pass, then dim your lights twice.
- Car 9, section 11. Just my husband.
- Car 9, section 11? This way please.
- Thank you. My husband doesn't like to be helped.
[...]