there were also ore trains from Phila. to Bethlehem, which were solid hopper trains.

I second that Butch. I lived right next to the Bethlehem branch. Sometimes they would drag gons full of scrap metal at night behind the ore. From what I remember when they ran with the extra gons, they added a 5th unit. The best were the extras that would run during the day from time to time, all hoppers.

This was a topic brought up at Tom's "Realizing the Reading" seminar at the Malvern RPM a few weeks a go. There was a unit train of mostly B&O hoppers picked up by the Reading at Hagerstown, this ran north through Tamaqua then on to the Catawissa branch handed off at Rupert to the E-L. I had a lengthy discussion about this with a veteran modeler who photographed this in action with the mid-train helpers in and around E. Mahanoy Jct. and Lofty. I did find a photo in one of the Kantner brothers books of these mid-train "geeps" in action and from what I gather was a real treat to watch during its operation!

A unit train is a train where all the cars or blocks of cars are traveling on the same waybill from origin to destination. They weren't used until the 1960's and weren't really common until the 1970's.

The Joanna ore trains were probably unit trains, but the coal trains and the Star were probably not unit trains (I have seeen pictures of the Star with various types of cars on it). They were trains of the same commodity and the train might have the same origin and destination but the cars were moving on individual waybills. The P&R/RDg ran solid trains of coal from the beginning.

You're correct that the Star wasn't a Unit Train. But, from the mid-60's on the Reading ran lots of Unit Coal Trains, to Philly area users like PECO, to interchange points like Rupert and even to Lehigh Portland Cement in Evansville. On some occasions, several blocks of cars (identified as Unit Trains by RDG) were combined together if traveling in the same direction.