I have recently run across a number of references to the 1500s as being "T-54 class." I don't think I have ever seen such a designation on any Frisco reference unless it is supposed to be shorthand for Tonnage Class 54 (and weren't they Tonnage Class 56?). Did the Frisco ever use such a designation internally, or is it picked up from an assumption about the way the Frisco worked. I know many other RRs used such designations--the MoPac being an example near to home (e.g. BK-63 for their 63" driver Berkshires)--but I haven't run across such usage in or on anything Frisco.
The designation appeared as “Sub lettering”, which appeared below the Frisco letting on the cab. As built, the 1500’s were T-54. The Frisco also tended to include the booster engine TE in the tonnage rating. Take a look at the diagrams and early ETT’s
Changes to BP and driver diameter altered the 1500's TE. Delivered with a BP of 200, which the Frisco later boosted to 210. Driver diameter was increased to 69.5. Using Cole, here are the differences: BP Stroke Cylinder Diameter Driver Diameter MEP TE 200 28 28 69 0.85 54084.64 210 28 28 69 0.85 56788.87 210 28 28 69.5 0.85 56380.32 I have been in both T54's
That was the impression that I have always gotten from talking to people and reading--at least until recently when these T-54 references started popping up. They seemed to be that people unfamiliar with the RR confused the Tonnage Class designation (Karl's chart neatly shows why some of the diagrams show T-56) with the class designations used by other roads (such as the MP). If anyone has Frisco documents (other than the diagrams) referring to the engines by their tonnage classes, I'd be interested in seeing them. Karl refers to ETTs, but I am unfamiliar with that reference.
The Employee Timetables (ETT's) include a Locomotives By Tonnage table in the Special Instructions section. I don't believe that this "classification" was ever used to identify a locomotive or the "class to which it belonged, per se; that was done with the number, e.g., 4000-class, 4200 -class, etc. I don't believe that one can equate the Frisco's T-number to other roads' systems, e.g., K-4, L1, M1, Y-6, Ja, etc. Soon after the Memphis Road merger, an attempt was made to develop a standard locomotive taxonomy, which would be displayed on the coonskin number plate. It was known as the Hancock system. Will have to post that system later. Search the site for the string, "hancock".