If these are the grey and black smooth-sided cars (Rivarossi calls them 1930s cars), then use the Kadee #508 conversion bolsters. These maintain the Talgo-style mounting that comes with the car, but allow for closer coupling with Kadee couplers. If you're not having problems with derailments, these should work fine. Their installation requires a slight modification to the trucks, but nothing difficult. If you go this route, you'll also need to replace the wheelsets with Kadee 36" passenger car wheelsets, in order to maintain proper coupler height. The cars will also track better with some additional weight. As Jim notes, body mounting the couplers is an option only if you have wide (at least 36") radius curves. If you're looking for a similar style of car, but shorter, check out Concor. My Walthers catalogue, 2005 edition, shows a complete train consisting of baggage, rpo, coach, diner, sleeper, vista dome, and observation, all 72 footers, about the same length as the Athearn heavyweights. Unfortunately for you, the CNR version is in the green and black paint scheme, although they also offer the same cars painted for VIA, and undecorated versions are available. Interestingly, the baggage car is patterned after CNR baggage cars built by National Steel Car, in Hamilton, Ontario, and is a very good representation. These cars also have Talgo-style couplers: if you want to convert them to Kadees, use the #508 conversion bolsters. The wheelsets are metal and do not require replacement.
Wayne