Having in my career been on Amtrak trains even in the cab at times when 13 fatal crashes occurred, I wish that the horn was much louder and that the "No-Whistle Zones" were eliminated. It take a terrible toll on the crew when an injury or fatality happens. Amtrak currently offers the crew relief if they feel they cannot proceed after a fatal accident. I gave up counting the grade crossing accidents where there were no injuries. I have seen all kinds of accidents, from pedestrian to loaded semi dump trucks. The first was a fatality the dump truck ended up with 3 minor injuries in the cab of the engine. The two examples cited occurred at 79 mph.
I could tell you railroad stories from now until the next "Super Moon". No whistle zones do not stop grade crossing accidents, I even think that train vs pedestrian accidents increase. Just for the record I grew up within 75 feet of the PRR mainline on the south side of Chicago and currently live in a suburb serviced by Metra within 1/4 mile of a station and the horn is music to my ears.
Sounds like the answer is grade crossing separation, not louder horns.
I live in downtown Chicago, where we quite possibly have the loudest fire truck sirens/horns in the country. Despite this, people still act like the don't know what to do when they hear an emergency siren coming at them. I don't know what their decibel level is, but I have to plug my ears when they go by to protect my hearing. You can hear the siren from blocks away, but all that does is train drivers to ignore it until it's right on their ass because you can't tell if it's on your street coming at you, or two blocks over.
As for grade-crossing collisions, how many of them would really be avoided by louder horns? When someone's racing around the gates trying to beat the train, it isn't the loudness (or quietness) of the horn that's the factor, it's the (perceived) time savings that they will get by not having to wait for the train.
Even in quiet zones, the crossing gates have bells on them which alert people. Some even have smaller whistles attached to the gates themselves that blow when the train approaches, so the train doesn't have to use its whistle. I've never been at such a gate when a train has passed, so I don't know how they sound. However, I'd be in favor of some kind of advanced technology at the gates that makes the whistle sound louder as the train gets closer. Engineers are still allowed to blow their whistle, under any circumstance, if they feel that there is some safety-justified reason to do so. But that still requires the pedestrian to have their head out of their anal cavity, and not having a cell phone on one ear while plugging the other (to tune out the loud noise of the train coming by), or kids with really loud iPods and earphones, etc.
Trucks getting stuck on crossings because their trailer is too low...can't really help that with a horn (unless it's of just the right frequency to resonate with the trailer and the rails and allow the trailer to levitate just enough to...okay, not likely to happen). Then there are those who cross the tracks but can't clear due to traffic in front of them. That's just plain stupidity. Sometimes there's an intersection immediately after the crossing, and the traffic lights have not been programmed to link up with the crossing gates. Again, doesn't excuse poor driving habits (entering a crossing without being able to clear), but there are changes that can be done to mitigate that problem (like all directions changing to flashing red, except for the direction that's trying to clear the crossing, which gets a green so they can get the hell out of there).
Point being, with as loud as train horns are, people really have to try to not hear them, and the answer to that isn't louder horns, it's fixing the real problems (as varied and numerous as they are), because making louder horns will only drive more people to demand quiet zones (which you're not going to get rid of, no matter how much you wish), and make others find more ways to tune them out (which we're still not going to avoid).
At some point, doing something in the name of increased safety becomes counterproductive as people look for ways around it.