There are various options depending on how the factory has applied the numbers. The easiest were Kader produced models for Mainline, Replica and some Bachmann where enamel thinners will remove them; just watch out out that lining is not damaged. Has also worked on some recent Hornby models.
A seemingly crazy option is simply to rub directly on the digit with a wooden cocktail stick or tooth pick. A version of that is use T-cut as well but I have have never had to.
A further weird version that has recently been found is to rub over the numbers with a Sharpie marker and then wipe both the marker and the numbers away with a brush soaked in acrylic thinners.
An old simple mention that is properly difficult today was to use a typists eraser pencil from the days of typewriters