painting pre-war loco?

Without a picture, its impossible to judge the condition.

Anyway, it's your train, and you should do as you please.

I prefer to leave things in their original finish, scratches and all.

I've repainted most of those i own that are badly scratched, etc.  I strip the paint and prime them.  

On something like that, with scratches, I would first use Rusoleum gray auto body primer rattel cans.  It is a good paint that dries thick, fills small scratches and sands well, then sand carefully with #400 afterward until smooth.  if you have scratches visible through the primer after spraying, use Bondo Auto Glaze filler, the type not labeled as "professional' grade (professional grade is the two part you mix, not is just a red good you spread on and good enough for small scratches right over primer).  then sand with 220, 320, 400 and smooth.  On tinplate or very smooth locos I use satin or semi-gloss black.  

Paints that match our trains are available from several dealers, such as Hennings and George Tebolt. Cans and jars for airbrushing, and spray cans are available. Many hobbyists use brake fluid to strip off old paint. There are also a few people that do custom paint jobs if you want to go that route.

Larry