I have used a lot of the Woodland Scenics dry transfer letters along with other railroad numbers. I have had good luck just rubbing them down on flat paint then sealing with a flat finish, but I suspect they are just riding on the high points of the surface and not really down good. However, they seem to be staying OK. Problem here is, you have to be dead on where you want it our you have to do it over.
I have very good luck rubbing them off on clear decal sheet, cut them out, then apply them just like any decal. Works great. If I am having a problem with the transfer lifiting or coming off the decal sheet, I will seal them with Krylon or Microscale decal film. Works great.
For, say, serial numbers on an aircraft tail, it is better to put them on as individual numbers and not try to put them all in a row on the decal sheet and cut them out as one long number. I could never get them lined up right on the decal sheet. I can line them up better individually on the model.