Hi, Jim,
The whistle signals all had meanings, of course .... think of it like Morse Code. The various signals were meant to let the Train Crew, back in the caboose or on the car tops, know what was happening on the head end. Most railroads had a rule, for instance, that the train crew (Brakemen, Conductor, Flagman) had to watch as their train crossed public roads. The reason seems to be that it gave the company more witnesses in case of an accident. Thus, the long-long-short-long, or long-long-short-short would alert the rear end crew that their train was approaching a road crossing.
In these days of cabooseless trains with two-person crews, it would seem there isn't much need of this. I would rather see your Engineer use the time-honored signals, but I suppose almost anything that alerted motorists or pedestrians would now work ok. That would be up to the railroad's rules examiners. However, you will find that many people have become used to the standard crossing signals, that it could make a difference. I am surprised at how many non-railroad folks can tell me what the crossing signal is (long-long-short-long.)
Skip Luke
former Train Dispatcher, IC RR, WP&YR, BN RR.
Engineer, GL RR.