Most of the people have a good laugh after being surprised.
The guys in the car pull up behind a bunch of cops in a line, with their backs to the car. When the train horn goes off, one cop starts to pull his gun, but then they seem to have the most fun of anyone in the video.
Many people just treat it as a loud noise, but from their reactions you can get a good idea of who know that the sound represents a real train.
HornBlasters Terror On The Streets Deleted Scenes Train Horn Pranks Cops - http://www.youtube.com/watch?v=zzqrZOq928w
Or for those who can see it-
[youtube]http://www.youtube.com/watch?v=zzqrZOq928w[/youtube]