RR air horns: About 50 yrs ago I worked summers on a section crew for the NP Railway.?

We worked in a narrow valley where three major lines converged: NP, GN and UP. I could tell which line was coming through by the distinctive sound of their air horns.

For instance The Burlinton Line was a joint NP/GN RR and occasionally a Burlington loco would come through with its single tone bass horn.

I cannot find information on which RR used which horns (the chord they played).

Any help out there?