“As soon as they see a pressure drop the pumping stations shut down.
It’s a 36”(?) diameter line, about 50 gallons per foot.”
In this day and age there is no excuse to loose anywhere near this volume before shutting down. No doubt they have flow meters at both ends and a computer can compare the two readings instantly. They should have electronic valves every so often so that they can even trap most of the fuel in the pipeline.
Not easy to stop a thousand mile length of nearly incompressible fluid.
Guessing that the pumping stations are 50 miles apart, depending on elevations... 50 gal/ft...
How far apart would you like your isolation valves?
They would require a massive accumulator.
How quickly would you like the valves to shut?
All that added surface area, subject to the same elements as the line, or worse; may not be any better off.
And the added complexity...