I think it depends on what model you're starting with. I used a silver Sharpie and a straightedge to paint the braces and trim on this Deluxe Schneider trailer way back when:
The key is to stick to surfaces that stand off from the walls of the trailer so the Sharpie doesn't bleed. Practice first, if possible. It's also important to seal the ink afterwards otherwise it will wear off with handling. Just be sure to avoid alcohol based finish coats, like Dullcote, since they will dissolve the ink and make a huge mess. I used Microscale flat finish and it has held up well for over 10 years.
If you're starting with a smooth side trailer (since the Deluxe trailers are no longer available) and you just want to simulate the bracing, decal stripes would be a good way to go. That's what I used on the lock rods of the above trailer, for example.
As it happens, I have a few of the Deluxe trailers left over from a "scratch & dent" set they used to sell and I'm looking to do some ex-Triple Crown trailers like this:
http://www.rrpicturearchives.net/showPicture.aspx?id=5207964I first need to source some ghosted Triple Crown decals though. (I'm also planning to bash the spine car in the above photo using an N Scale Kits set as a starter. So many projects...
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