Wiktionary
vb. (context transitive idiomatic English) Use a flag or similar kind of signal to get someone's attention.
WordNet
v. signal to stop; "Let's flag down a cab--it's starting to rain"; "The policeman flagged down our car"
Usage examples of "flag down".
But a lot of local telecoms turned FLAG down anyway - at least at first.
A wave of a bag of silver was all that was needed to flag down a passing fuel truck, and soon his tanks were full.
A poet might have seen something dramatic and beautiful in those spider lines cleaving the air, but Bush merely saw a couple of ropes, and the white flag down in the launch signalling that all was clear for hoisting.
When she looked out the windshield, she saw him trying and failing to flag down a taxicab.
I can't think of another reason to flag down a freight train on a night like this.
Once you reach the track, you should be able to flag down a passing vehicle.