I saw an old tip recently regarding using sew-on snaps to replace missing Lionel brakewheels. Reproduction brakewheels generally are available, but sew-on snaps work well, are readily available at any store that sells sewing supplies, and they’re cheap.
As an example, I purchased the Lionel 652 gondola on the right at a train show for a very low price. It was missing the trucks and brakewheels, the frame had lost most of its original paint, and it was pretty beat up. I repainted the frame, put two cast-off postwar trucks on it, and glued #1 sew-on snaps in place of the missing originals. The brakewheels make the car look more complete and less like a beater. It’s still not a prized piece but now it’s a useful car, allowing me to run mixed consists of Lionel and Marx cars.
Prewar cars often used grossly oversized brakewheels; a #4 snap is a reasonable approximation of those. You can get mixed packages or packages of all one size.
The snaps generally come in black or nickel finish. I used nickel, but black may be a bit more realistic if realism is what you’re after.
In my example I used a Lionel car, but of course, if you have similar cars of other brands that are missing brakewheels, you can use sew-on snaps on those just as well.