In the olden, golden days of yore, when horses pulled boats along the canals, speed was of the essence.
The faster you could go, the sooner you could get your next load, the more money you could earn.
Wherever there was a bridge the horse couldn't go under, it would have to be unhitched, walked around, and rehitched – time consuming!
Any small bridges had a slot for the tow rope to pass through. (See photo.)