One suggestion: my railroad is the "Minnesota, Sioux Lake & Western", so I use initials on the diesels or a logo with "Sioux Lake" as the colloquial name. If you can use a portion of the overall long name in a similar manner, then do so. That way, you have 3 options for lettering: initials, logo + short name, and full name, where different situations require different solutions. I have used Avery clear non- glare address labels for small letters- only decaling, using a free copy of "railroad gothic" font I downloaded, or for more modern purposes, Arial bolder font. In either case, you can make the numbers and letters any size, from 6 point to 14 or larger, without losing clarity.
You can use these fonts in MS Paint and place them centered in a box by moving the separate image elements around or double-spacing lettering, to increase visibility when placed on a side of a steam or diesel cab.
Check out Decals.com for bulk inexpensive decal paper, that you custom print yourself, then seal with clear spray, allow to dry and then cut-soak-apply. Testors also has a decaling kit.