I don't see anywhere on the TTX website or wikipedia explaining what the X in TTX stands (or stood) for. But in your instance, as part of the company name, the X doesn't represent the last character of a private owner reporting mark (all private owner reporting marks are four characters, and the last character is always an X). It's simply part of the name.
TTX operates cars under many different reporting marks. In some instances, as Curt mentions, the X does end up as the last character in the reporting mark, such as the CTTX mark found on many autoracks. But any number of car types and classes under TTX ownership bear reporting marks not ending in TTX. All of the boxcar groups (ABOX, RBOX, FBOX, TBOX) and all of the gondolas (GONX, GNTX) obviously don't have TTX in the marks, and there is a whole group of cars with marks formatted TT*X rather than *TTX. In fact, historically there are 24 reporting marks in each format, with the * replaced by the letters A-Z, excepting T and Y.
Jim
