From the Passenger Car List (Groups.io) October 2018:

I have acquired a number of color slides of Mo Pac heavyweight cars and notice something a bit unexplained on them. The cars are in the EAGLE grey/blue scheme, and usually on the right end of the side is a letter…B, C or D, sometimes in a circle. Neither the cars painted Jenks Blue nor the sleeping cars have this. What did the letter represent?

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The A-B-C-D speed classification system was put in place in 1948, replacing an earlier A-B system which basically differentiated between passenger car trucks and freight car trucks. As outlined in the MP employee timetables by August 1948 and thereafter:

Consist “A” – any type locomotive with one or more cars equipped with freight car trucks.

Consist “B” – any train pulled by steam, or any diesel passenger locomotive powered train with one or more conventional weight passenger cars with high center of gravity

Consist “C” – Diesel passenger locomotive powered trains with streamlined high center of gravity cars which are equipped with electro-pneumatic brakes, tight lock couplers and roller bearing trucks (whether or not any low center of gravity cars were included in consist.) Passenger cars MP 6330-6336 (coaches), sleepers Shrine Tower, Beacon Tower and Temple Tower, and diner 10242 are streamlined HIGH center of gravity cars. (This list was as of the August 1948 timetable; all of the above cars were modernized heavyweight cars.)

Consist “D” – Diesel passenger engines with streamlined, lightweight LOW center of gravity cars only. MP passenger cars of the 700 and 800 series, Sleepers with “Eagle” prefix or “River” suffix, and Rock Island sleepers 504, 505, 508, 510, 511 and 512 are streamlined LOW center of gravity cars.

Cars specified in the C and D classifications varied with the year, the above lists were from an August 1948 employee timetable. The employee timetable or special instructions also prescribed the allowable speed for each consist type on tangent track, on curves, etc.

Bill Pollard