I have the book "Union Pacific Railroad System Employee Timetables February 29th 1948", published by the UPHS.

The Challengers were permitted 65 MPH on passengers assignments on the Northwestern District. The Southwestern District had been dieselized by then, so there are no speed limits published for steam from Salt Lake City through Los Angeles. On the Wyoming and Utah divisions MAS was 60 Passenger, 50 Freight.

The 4000s were cleared from Granger to Pocatello via McCammon. The passenger and freight MAS limits were identical: 50 MPH. Green River to Ogden was 45 Passenger - 45 Freight. There were no speed limits shown for the Wyoming Division, because they hadn't yet been displaced by diesels, Ogden to Green River, and moved east for operation over Sherman Hill.

The 80 MPH "design speed" was, in fact, an over-design to insure against machinery failure at the 45-50 MPH speeds where the engines were going to be operated. There was never any intention to wheel passenger trains at 80 MPH behind the 4000s. Most passenger assignments behind 4000s were main(troop)trains.

Locomotives as well as buildings, ships and bridges were all overbuilt in that era. Metallurgy was not what is is today and structural members were routinely built 1.25 to 1.5 times as robustly as they would be made today.

Ed Dickens is on record stating that he intends to operate 4014 in the range of 40-45 MPH. The majority of video that I saw of the Mid-West tour shows the engine in that speed range.

UP already had an express passenger locomotive in 844.