Reply to "What does the X stand for in the TTX ?"

CSX is a corporate name. The reporting mark is CSXT.   When I worked for a railroad, cars with railroad reporting marks were paid for time and mileage and ther use was controlled by the car service rules. Cars with reporting marks ending in an X only got mileage payments and their empty movement was managed by the owners or lessors. There are other special last place marks like U and Z for trailers, containers and chassis.  Reporting marts do not need to be four letters. They could have 1, 2, 3, or 4 letters followed by one to six numbers.