This photo shows several Department of Defense owned flatcars (DODX 40129 and others) loaded with M1 Abrams Main Battle Tanks bearing the markings "3I 3-7 CAV" indicating 3rd Infantry Division, 3rd Squadron, 7th Cavalry.
Each DOD owned railcar is color coded to denote its type and capacity.
DODX 40000 series are green, with 150-ton capacity, 68 feet in length. These will hold two M1 Abrams.
DODX 41000 are red, with 100-ton capacity, 68 feet in length.
DODX 42000 are yellow, with 80-ton capacity, 89 feet in length.
Consult TEA-PAM 55-19 for loading and tie down instructions.
Thanks to Billy Grimes for the information about markings of DOD owned railcars.