Most Eastern railroads use NORAC rules. The FRA requires ditch lights on when running more than 20 mph over grade crossings. And the 'flashing' feature appears to be quite popular by most of the railroads there. They usually do the alternating flash when either the horn is sounded or the bell is ringing, and keep this up for about 30 seconds after the engineer stops the bell or horn.
Western railroads do not seem to use the 'flashing' feature that much, but I have seen BNSF units like Gensets with them(maybe they were set up at the factory with them and no one has changed it).
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
According to this link they are RSL-3LR: http://www.trainorders.com/discussion/read.php?2,341455