Companies that end in X are leasing companies.. I.e. HLCX, TTX, EMDX, etc...
Car loads are done on car by car basis - a contract isn't required for railroads to ship over another railroad. Its done by waybills, etc..
The big railroads have been leasing engines because they are in need of power. Helm Leasing (HLCX), FURX, LLPX, EMDX, are just a few of the locomotive leasing companies. This is done by contract.
Cars are leased to companies too. TTX/TTLX is just one - they lease flat cars, auto carriers and the like. GATX, GACX, etc. lease tank cars and bulk hoppers and there are many, many more.
P.S. As additional info to clarify my opening sentence, CSX's full reporting mark is CSXT -- they aren't a leasing company.