straight from the Federal Railroad Administration:
The maximum volume level for the train horn is 110 decibels which is a new requirement. The minimum sound level remains 96 decibels.
Like sirens, they're rated @ 100ft
If memory serves me right, the AirChime K5LA, the most common horn in north america is rated 115db @ 100 ft.
at 10 ft they can reach 120+db. Good ruse of thumb, if you're doing a static test of a locomotive horn (stationary test), wear muff style hearing protection, even if you stand behind the horn. I've had my ears "cleaned out" by my Nathan M3 inside a pick up cab with all the windows open, with the horn mounted on the roof, just before a run by. If you're sitting at a crossing and worried about the horn of a passing locomotive, you'll be fine without any hearing protection for the brief time it takes for it to pass by.