Where the heck is that loud airhorn sound coming from?

 

I took this photograph today at work, as I was heading out on my morning lunch break.

 

I had just turned on to Chestnut Avenue in Glenview Illinois from Johns Drive, when

I head this loud "Obnoxious" air horn blasting in the distance. Although the railroad -

crossing gates were still in the up position, a convoy of self propelled railroad vehicles

were soon spotted crossing Chestnut Avenue, enroute to a job site somewhere on the

Metra / Milwaukee District north line.