Some of the TTX reporting marks usage have changed and been re-used over time.
In the 1960s, PTTX was a flatcar with an adjustable bulkhead for wallboard service. (Eventually they got their bulkheads removed and converted to various OTTX, TTMX, TTHX, etc. types.)
Currently PTTX is 85’-89’ pipe service flats.
TTAX used to be 89’ all-purpose trailer/container intermodal flat cars suitable for “circus-style” loading at TOFC ramps of 2 40’ trailers, or 4 20’ containers. Today TTAX is used for articulated intermodal “spine” cars.
TTCX used to be 89’ container-only intermodal flat cars. (2x 40 or 4x 20 containers) Later TTCX was used for 60’ open deck container flatcars.
LTTX used to be 89’ trailer flat cars suitable for “circus-style” ramp loading of two 40’ trailers. Today LTTX is 89’ flat cars with bunks for long telephone poles.
RTTX used to be open tri-level autoracks. Later RTTX was used for 89’ all-purpose trailer/container flatcars with hitches at each end suitable for overhead loading only.
BTTX was once open bilevel autoracks. More recently BTTX was two-unit articulated enclosed belevel racks.
etc.