I do kinda wonder about these "major snowmaking lines" that connect the main water source these mountains use to their on-mountain systems. Killington tapping into the Woodward Reservoir comes to mind. Okemo's big pond just outside Ludlow that takes from the river is another. Major pipes like these must be pushing 30+ years old at most resorts and would be a huge undertaking to replace I'd imagine, tearing up roads and lots of digging.
Killington's line from the reservoir is PVC, gravity pitched. Most resorts do not want to pay the expense for the quality pipe so this is quite common.