I, too, model the transition period between 1945 and 1960, primarily the Burlington Route, Great Northern, and Northern Pacific. I also lately added a 1955 vintage Milwaukee Road Hiawatha. Most of these are diesel-powered. But I also run heavyweight trains pulled by steam. As the prototype did, it sometimes became necessary to use steam locomotives to back up diesels. I run four types of steamers on passenger trains: For short hauls, Burlington 0-1a 4-6-2 light Pacifics; more often S-4 4-6-4 Hudsons; a 4-8-2 B-1a Mountain; and the more commonly used for highspeed freight and passenger, as well as excursions, an O-5a 4-8-4 Northern. Although I haven't numbered my Northern the same, the Burlington Route ran passenger excursions with O-5a #5632. My steam locos are all superdetailed or kitbashed from Mantua/Tyco Pacifics or Mikados, and are re-motored using the no-longer produced Canon coreless motors. The Northern was made from two Pacifics; the Mountain is a Mikado with the forward frame and boiler spliced from a Pacific for the 4-wheel lead truck. But anything can be used, as the prototypes mixed equipment as needed for excursions, including regular streamlined and heavy passenger cars, modified open gondolas, and caboose/waycars. So far as train length is concerned, I am pretty well limited to four or five passenger cars in a train, depending on the length of the cars (72 or 85 ft. models).