This is from the BNSF perspective, but might hold true for CSX too. From what I've observed, railroads would do something like this for a couple of reasons. One would be, as you mentioned, some specific service.
The other reason would be for fleet or service protection. I'm told this is how BN/BNSF operates, such that the railroad owned cars would ensure that contracted deliveries were upheld. So if a utility purchased/leased car went out of service, these extra cars would be inserted into the trainset to make sure a full train was in-tact. In BNSF's case, they would often run entire trains initially as well, and I'm not entirely sure of the motivation for this. I've been told that BN then BNSF emphatically did NOT want to purchase their own cars, but that's an odd proclamation for a corporation that always seemed to be buying them. I would guess the utilities bought the minimum amount of cars to meet their contract and it just made it easier on BNSF to have an extra train or two in the loop. The beauty of Autoflood II's and similar designs is that they could fill in for either hoppers or in place of rotary gons so they were very versatile. BN's fleet of Bethlehem rotary-equipped steel triple hoppers performed a similar function before the onset of aluminum cars and stuck around through the early 2010s.
So to tie this back to CSX, these cars could have been a small-ish order of "jack of all trades" filler cars for their coal operations. That is of course just a guess on my part.
