So, I think the "Cattle Car" is a terrible idea, mounting a tank cannon on a almost completely unarmoured transport carriage is a pretty terrible ideas for many reasons. Can you imagine a cattle car with 15 horses in being rattled by the recoil of a 75mm gun? Any return fire trying to silence the gun is effectively aimed at your troops, any ammo incident blows up your guys. If you're going to the effort of a train with multiple carriages you might as well go to the effort of having separate transport carriages and highly explosive carriages.
For versatility and logistics you probably even want separate infantry and horse carriages, although I haven't checked if there is a rules advantage to having a single 40T carriage over 2 20T carriages but a 40T locomotive pulling 6 20T carriages - something like weapons car, infantry car, horse car, infantry car, cargo car, weapons car might make a decent military asset for a backwater world. Might behove me to check the towing rules as I don't know them off the top of my head.