I actually have a nine car train of the ConCor 72’ smooth side cars that runs very well and looks good.
I did do some work to them. They were the older ones with the push pin truck mounts, not the newer ones with the screws.
Removing the roofs is not hard and can be done without damage to the tabs, simply pry/push them with a small screwdriver until all are released, then slide the roof/window out.
They do need weight, no question. And I too body mounted the couplers and replaced the trucks. I used MDC plastic trucks (no longer available) and Intermountain wheel sets.
I also “frosted” the glass rather than add interiors.
Additionally, I cut down the molded on diaphragms and installed American Limited working diaphragms and ajusted the coupler positions to provide touching diaphragms and near prototype close coupling and car spacing.
In replacing the trucks I added styrene pads and drilled new mounting holes for 2-56 screws.
My investment was still WAY less than these current expensive RTR cars, most of which I would have to work on anyway since the poor design and coupler mounting that leaves the cars too far apart and the diaphragms not touching. $50 to $80 and the diaphragms don’t touch and work - they are kiding right?
I can make $15 passenger car look and run great for an extra $15 - those not willing to learn how or do that are welcome to spend $80 a piece if they like. It is a hobby full of choices.
Most of my other passenger cars are Athearn, both streamlined and heavy weights that have been super detailed and fitted with correctly spaced working diaphragms as well.
Personally, all the detail and scale accuracy in the world is spoiled by widely spaced long cars squeaking around sharp curves with the diaphragms a foot apart.
I will take my selectively compressed by equally detailed cars with prototypical car spacing and working diaphragms over that any day.
I run my cars on 36" and larger curves, but they