This tutorial will show you how make the cab numbers completely transparent. Your numberboards will still work fine however. This is useful if the locomotive you're repainting has the cab numbers in the wrong place and you have to paint your road numbers on. i.e. my Boston and Maine GP9 repaint:
Here are the tools you'll need:
GIMP
GIMP DDS Plugin
Now first you need to get the number textures exported. You'll find them in the textures folder:
Although they won't always be named as such, it will likely be something similar.
Open up your number texture in GIMP. You'll see that it's divided in two (or in some cases three) pieces:
Please note that your cab number will not always be on the left side. It all depends on how the locomotive was set up. But it shouldn't be too hard to figure it out.
Next, use the Rectangular Select Tool to select the cab number:
Up in the toolbar, got to Layer-->Transparency-->Threshold Alpha:
This will open a new window with a slider labeled Threshold. Move the slider all the way to the right so the value is 255:
And like magic, the cab number has vanished:
Finally, when you save it, make sure to use DXT3 compression and select Generate Mipmaps:
Now just repeat the process on all the other numbers and import them back in over the old textures.
And that's it. Now your cab numbers will be completely transparent but the numberboards will still work fine.
I don't know why I didn't do that to begin with. I guess I'm slipping. 
