Rrman, others have adequately covered the requirement of the rule for sounding crossing warning whistle signals, except that Federal regulations require it to be sounded for not less than 20 seconds nor more than 30 seconds each time.
Things in play affecting what you are hearing:
- Yes, at least some of the locomotives have the modulating air horn control valve, usually operated by a pull-lever mounted on the control stand, or the same type of lever mounted in the cab ceiling and activated by a rope attached between the lever and the control stand.
- If the light whistling is common, then the railroad managers, at their own peril, are not putting forth the effort to verify whether the rule is complied with and correct the attitudes of any employees who are deficient. There are -- and always have been -- Engineers who are irritated by the whistle and do not like to sound it long and loud. However, there are also vast herds of plaintiff attorneys who have become much more sophisticated in using disregard of railroad rules to portray the railroad and its employees as being cavalier and unconcerned about safety of the public, when playing to a jury which has the power to, and often does, award large sums of money to people who are injured by trains even though it is primarily their own fault that they were injured or killed.
- If a rule is badly designed (and this one is an excellent example of government employees crafting a rule with many complicated aspects) then full compliance with the rule will get it changed by natural processes. In this instance, if the Engineers fully sound the whistle as required and, say, it is in a residential neighborhood, the residents will complain to their elected officials, who, in order to have some peace, will appropriate the funding necessary to create a quiet zone* and the whistling, along with the angry letters and phone calls from constituents, will stop. The reason this complicated government rule got into the Code of Federal Regulations in the first place was to stop the angry letters and phone calls that were coming to members of Congress from constituents complaining about loved ones injured or killed by trains which -- allegedly -- did not adequately warn of their approach.
I hope this helps in understanding what you're hearing. Some employees are lazy and some officials are lazy.
* In an unusual example of fairness, the local agency requesting a quiet zone must pay all of the costs to upgrade crossing protection. This includes any active warning devices (signals), lane barriers, signs, etc, and is hugely expensive. The railroad is responsible for none of the extra cost of quiet zone road crossings, except for the roadway in the crossing and a crossbuck at the crossing. For once, the railroad got a fair deal.
