In that era, UP's standard horn was the Leslie S3L, with Canadian qualified units getting the S3K. Leslie and Prime didn't use rubber gaskets in them in the original design. The latter Leslie RS series (the ones with the "spike" sticking out of the back of the diaphragm cap) did have a molded rubberish type gasket around the diaphragm edge. Leslie air horns have a bad habit of sounding horrible as they wear.

Freezing rain or snow blowing into the horn bells seems to be more of a problem than freezing from the drop in air pressure. That is why some railroads use canvas covers over the horn bells, or perforated metal "snow cones". The 4700 series Hancock air whistles did have a provision for an electric heat element, likely do to its design using much more air creating the freezing up issue. SP used an electric heater block wedged into the horn manifold on their snow units.