That is somewhat correct Bret. Actually a significant portion is 70# from the U.P. Brogan Branch, which was donated in the mid '70s. I ran the Heisler some on track laying trains in 1977, using this rail. This was in the area from the McEwen yard limit to Hawley Spur. There is also some 85# stuff at McEwen that came out of the U.P. Huntington yard.
The 133# rail did not get used after all, it is still stockpiled near the backshop. I think a lack of comp bars was partly responsible for this. (75# to 133# is a big jump.)
You are right about the 75# rail and switch parts from the old South Baked Yard. This was the first rail laid at McEwen Depot. The Frog on the car shop lead was part of a turnout on the inside of a curve and is an odd number, which is why it doesn't fit very well.