In looking at different steam locomotives throughout the years, I have come to the question, how long does a locomotive go before it is repainted? I have noticed that Reading and Northern 425 has had some paint "patching" done, but not a full repaint, why is this?

.

In service, indoors or out, most operating engines are stored indoors, or under a shed, right?

Depends on the paint, the prep work before painting and whether it is kept inside or outside. A good epoxy like Dupont Imron for example (if it is still available) will last for years and years if prepped properly. A cheap rust enamel paint on the other hand you will only get a couple of years out of if you're lucky.

The paint on Black River & Western 60 is for the most part the from 1991, save for a few odds and ends.