The majority of the lettering on my locos and rolling stock was done using dry transfers from C-D-S. Some of it was from their own output, but I also had custom lettering from them, too, with the artwork done by my brother.
Here are a few examples...
When C-D-S closed, it was bought by a small American company, which opted to do the lettering as decals, but the ones that I bought from them were so faint that they were useless.
I later returned to using decals, and was fortunate that my brother did a great job of doing the artwork for an order from a decal supplier (whose name escapes me), and I believe that it may have been the last set of decals that he ever made, as he either quit the trade or passed away.
Wayne
What you seek to do somehow reminds me of some car forwarding ideas that came around during the 1960s when attention started to be paid to prototype operations. One idea was a variety of colored thumbtacks for car forwarding and the layout owner would drill hopefully discrete and small holes in the rolling stock in which to stick the tacks
The other idea was a fairly robust small slip of color coded paper or maybe it was a strip of brass that would stay with the car -- on the roof of a house car for example.
I bring these up because the thought strikes me that there might be alternatives to having the loco number decaled or dry transferred to the roof or top surface of the locomotive, particularly since both decals and dry transfers need a fixative or clear coat over them to be durable.
The thumb tack idea likely would not work too great in N scale given the need for a hole, but it might:any given exhaust on the roof might be a place for a modest hole . Perhaps a U shaped piece of brass or tin or foil with the number on it would fit over the body of the locomotive like a saddle. The idea is to create something with the loco number that goes ON the locomotive but is not a permanent part of the locomotive. It would give the operators the info they need but leave the locomotive alone for photographs, model contests, whatever, where the number on the roof might look odd
At the very least is could be tried and discarded if it does not seem to work where as dry transfers on the roof might be harder to remove without damaging the paint job.
Dave Nelson
Unless the entire number is in a row on a dry transfer I find it very difficult to get the numbers properly spaced and aligned. Many people use decals but they do leave an outline which is visible so instead of using decals sometimes I use a rubber stamper which has numbers which can be changed.
I like them best. They do get old though like decals but I have had some luck nuking them to restore them.
N scale engines are small, and the engine numbers are really small. I have two GP40s with the engine numbers on the number boards, but not on the body. You need a telescope to read those numbers!
I am considering dry wall transfers on the cab roof to make id simpler, and have never used dry wall transfers before. Any tips or suggestions?