I want to start with the qualifiers here, I have almost no experience with painting model trains. I do, however have several years experience working in an auto body shop doing show quality work, so think some of what I know about paint prep there carries over.
As far as the primer goes, most are made specifically to cover flaws and build up thick, with the intention of block sanding the primer flat. This is going to be quite problematic for the small details we deal with in cast boilers and rivet detail. For automotive applications, a high build primer is useful, so the products I am familiar with tend to lean in the opposite direction for what would be needed for model trains. I'd start by looking for primer specifically made for plastic models, or that advertises 'no sanding needed' as these will be much thinner and leave details intact. If you can get away with it, I would use only a thin coat of a self-etching primer on bare metal and no primer on plastic.
On either surface lightly scuff the surface with a grey scotch-brite pad. Thoroughly clean the surfaces, with a wax and grease remover. There is product made specifically for this purpose, washing with clean water, then with mineral sprits should do the job. Do not touch with bare hands from this point, as the oil on your fingers can cause imperfections.
On plastic I recommend using an adhesion promoter, this is a spray that helps paint stick to bare plastic, apply then wait about 5 minutes before applying your paint of choice. I believe the 'fusion' pain has an adhesion promoter mixed in, but in my experience, applying separate products works better than using all-in-one types.
Follow the directions on the cans, but in general hold about 10 inches, apply several thin coats rather than fewer thick ones, and use long strokes that start before the piece and end past it. Follow instructions for time between coats as well, this should be something like less than 15 minutes or more than 24 hours. Before handling the part I would recommend giving at least 24 hours and longer doesn't hurt here.
This may be over kill for a beat up post war locomotive, but it works quite well on show quality automotive work.
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