Here are a few covers from my RR perfin collection.
The perfin in particular is W155, Western Pacific Railroad.
This railroad had smaller named railroads that operated around California. Here are covers of each, with perfin W155 affixed..
So, there you have it. One perfin (WP - W155) was used on 5 railroads: Western Pacific Ry (the parent company), Alameda Belt Line, California Central Traction Ry., Sacramento Northern Ry. and Tidewater Southern Ry.
Many other Class one railroads practiced this usage of perfins as well.
Anyone have any to show?