Now that we have moved on from National S gauge day to St. Patrick's day here is a little more information on the four SSA cars in the second picture. S Scale America rolling stock is made and sold by Des Plaines Hobbies. The cars are accurate and well detailed models of slightly more modern prototypes than those S gauge cars made by Gilbert, Lionel, SHS/MTH/ST, K-Line and AM. The cars are shipped with high rail wheels and couplers, scale wheels are included in the box. The cars must be handled with care so as not to damage small, separately applied details.
The red Ralston Purina car is a 53'RBL insulated boxcar. It is the only one with truck mounted couplers and will run on Gilbert track with no issues. Just couple it into any Gilbert train. The 50' Cotton Belt FMC DD boxcar comes with body mounted couplers. Even still, it will navigate Gilbert curves with no issues and could be coupled to a 40' boxcar with no derailing issues.
The next two cars will require some modification to use on a Gilbert layout. The C&EI 40' MOD boxcar and the 50' BAR SD boxcar both have body mounted couplers, no issue with them. The problem is they both have a lot of underbody detail parts that interfere with the high rail wheel flanges on curves. The smallest radius curves they would run on w/o interference was 33.5". The really simple fix was to install the scale wheels in place of the high rail wheels, completely eliminating the interference. With scale wheels they run fine on Gilbert track. The other solution would be to remove the interfering details, but I do not recommend that.
These cars are not expensive for what they are. All are available in a variety of road names and with multiple prototype road numbers. I got mine during the Des Plaines Hobby semi-annual 20% off sale. The cheapest was $36, the most expensive $44.