I have stated the reason for my preference for scale length cars on many occasions on this forum. I grew up on a commuter railroad and saw 6-10 trains passenger trains every day of my childhood from age 5 to age 18. Those memories are so vivid for me that less than scale length cars are a compromise I am not willing to make regardless of the curve sizes. An 18" car that should be 21" looks stubby to me, and I just can't unsee that. This is especially obvious to my eye when looking at the locomotives that are pulling the train which in the case of the '48 Century were originally EMD E7s.
Of course, not all heavyweight cars were 80' nor were all streamlined cars 85' in length, so I also always recommend knowing your prototype if you enjoy modeling prototype trains which I do immensely.
GGD offered 18" car sets for versions of the Super Chief, Broadway Limited, and 20th Century Limited. They ultimately didn't reserve well and were cancelled. It would have been nice to see them go forward for those who want a higher quality detail specific train. I'm not sure what compromises we would have made to make those "look" correct, but it would have been a fun design challenge. In reviewing the drawings for the current offering, I learned a lot about the prototype train that was really interesting including the twin unit diner where the kitchen and the crew dorm was in one car and all the tables in the other. Some of those 22 section Pullman Standard sleepers lasted well into Amtrak.
Since we all model differently, if you feel the look on the curves is more important than the scale fidelity of the cars, then that is a perfectly valid decision for your railroad. That is what makes this hobby great. There are no wrong answers.
Totally unrelated to the 20th Century Limited, but I think Lionel did a wonderful job with their Amfleet cars as they look proportional in terms of cross-section to length. This was achieved by scaling the entire car down to something less than 1:48. The HHP8 the set was originally offered with was also proportionately reduced so the whole train had scale proportions at something less than US standard O scale.
