Here's what NSK has to say about these -
These QTTX flat deck cars were built by Trinity from 12/1999 to 04/2000 and from 12/2000 to 06/2001. They are are numbered QTTX 131260-131364 and 131430-131459. They are designated as TTX class Class FSH70 and AAR Class F423. The center section deck is 70' long and has tapered ends.
This kit is extremely easy to build, as it contains only the three main parts (plus steps and bolster pins). The trucks are MTL 1036. Recommended couplers are MTL 1023/1025. Alphabet decals are used for the reporting marks, and the TTX logo comes from a Microscale spine car sheet (60-822). Other decals are from a data sheet for large capacity cars. As the weight is evenly carried via the span bolster to all four trucks, the car rides very well and easily traverses tight curves. Price $23.99.