I've painted a number of steam and diesels over the years, as well as freight and passenger cars. I was lucky to picked up several unpainted Lionel pre-production engines and have since painted most all of them.
It takes a lot of time and patience, but the end results is well worth the effort. Here are some before and after photos of just a couple I have completed.
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Note the Worthington feedwater heater on engine 5509 and the addition of an oil bunker on the tender.