Al,
I imagine for most model RR pikes that a 20 car train is a 'full length' train. I know in Free-mo we're blessed with 15-20' passing sidings but I think those aren't normal for most layouts.
Consider me in this group. My layout space is ~28 x 32 which I think most would call a good sized room. Even still, my sidings are just long enough for my 'standard train' (16 cars, 2 power units, caboose). There is a minimum 1-1/2 standard train lengths between sidings. Had I gone any longer in siding length then there would be less tangent track between sidings than the sidings are long! It would look mighty odd to enter a siding while the rear of the train is still in the previous siding.
I can't remember who, but one of the great MRR authors I read early on said "you need a lot of sidings to run a lot of trains on a single main." So, that's what I did. As such, the original RP momentum adjustment is quite satisfactory. If I did momentum and load I would never get to track speed before the next meet!