I am wondering if they are installing or taping into an existing line This is what it looks like to me.
From my perspective it appears that they dug a huge hole around the existing pipeline then shored up the hole with timbers and planking then laid a huge bridging timber over the pipe and supported it with the chains and rods as would be done in today's world by turnbuckles winches, excavators or a crane, Now that the pipe is supported they will probably clean it then install a saddle clamp with a nipple and Tee with a valve and piercing cutter connect the spur line to the branch of the tee the valve on the run then place the piercing cutter through the center of the open gate valve cut the hole in the pipe back the cutter out close the valve remove the cutter then cap the open side of the valve. More than likely there will be 2 valves one on the branch between the tee and the spur so that can be left closed until the rest of the spur line is completed.
I helped weld a 10" Tee to a 12" line one time. Only I wasn't told it was a hot tie on a live line until after we had finished welding the flange for the Tee to the line. Some of the insane things welders do in their lives.