Most of the rule books and safety books show the horse to the side of the track, with a chain attached to the horse shunting loop on the side of the wagon. Often the rules have a direct prohibition on the use of the main coupling for horse shunting.
That said, as JimC suggests, there are many photos showing the horse in the 4 foot with the chain attached to the main coupling. Not wise, if there is a bit of a slope to the track it is lasagne for tea.....
Now , If I was a Board of trade inspector, with a suitable military title and a large moustache I would create a set of rules for horse working that bridge. The horse must be out of the four foot and attached to the wagon with a chain long enough for the horse on one side and the wagon on the other side to be clear of the restriction when the move is started. So , 4 yards horse, 10 yards bridge, 5 yards wagon hook. Say 20 yards. The horse can then pull the wagon with a slightly dragging brake through the bridge without danger of men or horse being caught in the narrow bit.
The same idea could apply for captan working. This is a working set, one day I will find a use for them .
Something like this: