Either the Atlas cars or the weaver will be fine with williams passenger cars. The Atlas cars just have a lot more detailing...so it you're looking from closer than 2 feet, you might notice the difference. The weaver cars are comparable in detailing to the williams pax cars, so I think they are more approriate. Just my opinion.
The PRR Atlas X-29's I have here most are the plain X-29 versions . The REA express version..I have the recently released ones, and maybe 1 or 2 of the middle division ones.
beth