I received some information from my uncle on the per diem and pasted an edited version below:
"The prime example of the per diem game was RBOX a huge fleet of box cars leased to various carriers that enabled investors to gain per diem and most importantly depreciation.
On the operating side, the per diem game took place (until the early 1980's , late 70's when per diem began to be computed hourly not daily) at the interchange every night. All railroads had a per diem train designed to get everything possible off line, in our case west-bounds, just before midnight since everything on your line at one minute after midnight belonged to you for a full per diem day.
Our train was the MA16 which reported Hillside yard at 4 PM and was designed to get the east end west-bounds off at Fresh Pond late that evening and bring the PC/Conrail"s per diem train, the LI2 back to Hillside.
This created some interesting dynamics. The engineer/conductor wielded a lot of pull since they could walk the train, refuse to take anything they considered a cripple, pump up the air, double over. The umpire of the game was the CR, PC block operator at Fremont (Fresh Pond) who controlled the interchange and determined arrival times. You had to watch him since somehow time stood still if the LI2 was running late."