At night under the lights, the number on the 5 car will look like a black square. There's not enough outline to divide the black. It might look OK in the sun standing five feet back, but not so good at night.
Two easy ways to fix would be (1]) in-lay a red flourecent over the black. Don't use regular red unless it has a backing to keep the black from bleeding through. A dull red will look the same as black at night. (2) Cut some white reflective strips the same width as the white outline and cover the white. When the track lights hit it, the outline will light up and it will look like a 5.
Just trying to help.