John,
I like Accucraft, but I only have their N.G. Freight cars. They are huge! They are correct scale wise in proportion to the track gauge. I imagine the passenger cars would be ginormous, being that the real ones were 50 foot long, approximately.I also suspect they won't handle R3, or at least not well. Bachmann's passenger cars are about 19 inches in length, and longer than LGB's. The height and width may not look correct, you would have to put it up against the engine to see.
As far as Accucraft, the 1:20.3 freight cars are huge, and my R3 seems to be the absolute minimum they can handle. One idiosyncrasy: these cars were very tight initially, and needed " break in", just like a locomotive! The axles Needed oil and break in to roll freely, and the couplers as well. The couplerswere too tight initially, and would sometimes pull the adjacent car off the track, not pivoting easily on curves. Just oil and break in resolved everything, very free rolling now. I like their cars, but metal trucks and wheels, very heavy to go up grade.
Try a Bachmann and see, or bash or build.
Paul