That lettering can be found in several forms in various decal sets.

Speaking of cataloging stuff :-) I have long wanted to actually sit
down with my decal collection and catalog all the wonderful little bits
of lettering that can be found on decal sets that are useful in many
other situations. For example I recall the decal sets for RF&P PS-1
40' box cars (from whom?) had 'watch your step' lettering that was
placed under the ladders. A nice detail, and not uncommon.

I estimate if I did this by myself I would never actually build another
model because it would take me several thousand hours to do a good job
of it.

Tim O'Connor