Horns vary depending on the road you're modeling. All were different.
To find the appropriate horn for the railroad I am modeling, I contacted its historical society.
Good luck,
Richard
You can research several sites pertaining to the subject:
http://trn.trains.com/railroads/abcs-of-railroading/2006/05/introducing-the-horn-section
https://en.wikipedia.org/wiki/Train_horn
http://atsf.railfan.net/airhorns/
http://www.dieselairhorns.com/collection.html
http://www.soundtraxx.com/choose/choose4.php
If you make a Google search on Locomotive Air Horn you can narrow down your focus, then specify a particular locomotive/railroad you are interested in, then check back here once you have that information.
Good Luck, Ed
Can someone tell me what make/model horns were available for diesel locomotives in late 1959?
RicZ