Yes, I see Tripoli road on the linked map - it's shown going west to Rte 3. But it doesn't go through to Waterville Valley (on the map at least). It stops just at the west edge of the piece of the map I posted. From there to the valley it's shown as a dotted line. As for the phone line, I merely meant it's not shown on the map, not that it wasn't there. All the old AMC guides at least up to my 1966 version mention it.
Then there's mention in NH_Mtn_Hiker's quote of the "old Tripoli RR". Now that's a new one. I wonder if that's the dotted line down to Waterville Valley.
I think it's remarkable that the wire is still around and can be found after all these years.
To sum up, it looke like there's two old trails to find, not one.
NH_Mtn_Hiker: you mean 3950', not 2950', no? There's a little black square just above the junction of the 2 quads on that trail.