I'm making this post for a friend and follower who said he really like the way my number boards turned out on locomotives. So here ya go Dave.
Myself I do not like the way the number boards light up when the lights are turned on, most of the time they are way too bright and kind of diminish the headlight, so I completely blank them out. Generally the molded number boards do not come out even with the face of the cab or rear, they are recessed into the openings a bit and if you try to decal them with numbers, a lot of the times they exaggerate the convexed look. To be honest, there are a few different ways modelers have overcome this issue, but this is my way of doing it.
After the locomotive is completely weathered, I insert the window/headlight/number board glazing into the cab and in the rear of the loco. Then I mix up a small batch of 2 part 5 minute epoxy and very carefully use a toothpick and fill in the number boards. I keep applying it until it's flush with the outer surface. It's self leveling but I keep an eye on it until it starts to firm up.
Once the epoxy is completely dry, I paint it with a flat black paint, this will help to block any excess light.
Once the paint is dry I apply the proper number board decals that are supplied in the decal sheet. I also come back a bit later and apply a decal solution set the number boards in place. My choice is Walthers Solvaset.
The decals I use are ShellScale Decals, they can be had in several different styles, designs and scales. For this loco I used "N-117" which came with black number boards and 8" white numbers.
Like I said, this may not be for everybody, but it works for me.
Here is the address for ShellScale decals