An under-rider is a wire pennant with a poured socket on each end. A typical under-rider is about 700 feet long. It is shackled to the end or the main tow wire and runs under the lead barge to next barge in line.
Most often, light flanges are added to the winch-head on the towing winch to form a reel to store the under-rider. Some boats have a light duty hydraulic reel up on the boat deck to store the under-rider. On rare occasions for a one time tow, I have seen an under-rider laid out on the deck of the tug or the barge.
The Canadian-link. Is a manufactured product that can be bought off the shelf. It is essentially two poured sockets connected by three links of stud-link anchor chain. The stud is removed from the center link and replaced by a piece of heavy flat bar with a hole drilled through the center.
Typically, two layers of tow wire (about 700 feet) are cut off the main tow wire next to the side of the winch drum. The main tow wire is attached to the socket on one end of the Canadian-link, the piece of wire that was cut off is attached to the other end. Now the Canadian-link is in place set into the main tow wire. The level wind fair leads on the winch must be offset to allow the Canadian-link to pass. The level wind must be hand operated when the Canadian-link is stowed against the side of the drum (yes it makes a lump on the drum).
The last barge is attached to the end of the tow wire, the lead barge is shackled into the Canadian-link. In effect, the end of the main tow wire becomes an under-rider.
Many tugs in British Columbia have the Canadian-link. It is less common on American tugs.