I presume that we are talking three rail Marklin here. I would get yourself a Marklin Catalog, Contact Scott at Helmut's Hobbies with any questions.
Marklin released a three rail big boy in the early 2000's and may have offered their Mikado (Trix models) in a three rail as well. They currently offer PFE reefer sets and several US prototype cars/sets. They have also offered several versions of the F units in various road names and a California Zephyr Set.
I was a Marklin modeler for many years but eventually switched to two rail DC due mainly to the lack of offerings from them in the US prototype area. I can't answer any of your questions about 6021 compatability as I left the Marklin brand before I went digital. I am sure that Scott at Helmuts can answer your questions. Marklin gear runs great and is high quality...Enjoy
Guy
see stuff at: the Willoughby Line Site