Can't help entirely Stephen but I doubt they were tethered, cattle don't usually have the necessary halters to give you something to tie on to and I certainly don't see anything to indicate this in pictures of cattle waiting in loading docks. If modern road transport is anything to go by you'd simply pack them in tight enough that they'd hold each other up and if there weren't enough to fill the wagon, then as you've already indicated you'd use a moveable bar or partition to provide the necessary support. Have to emphasise this is mostly educated guesswork based on no experience of moving cattle by train except what I've seen in photographs but some experience from the farm I grew up on.