Open wire carrier...
The D&RGW had a Lenkurt 33A system (vacuum tubes, three voice channels) that went from Pueblo to Alamosa. It was superimposed on a copper pair, most likely the message pair, between Pueblo, Walsenburg and Alamosa. We typically used the copper message pair for this kind of thing, since the DS pair had so many drops at all the phone booths along the way, it was difficult to filter them all and get the carrier system to work right. Then the voice channels at Alamosa fed east to LaVeta and and west to Chama on the physical DS pairs, iron wires. The third channel was company PBX phone to Denver, as I recall.
Other parts of the D&RGW used Lenkurt 45A open wire carrier on the pole lines extensively for this same scheme...leap over the distance with the carrier, then feed two ways on physical pairs in sections 50 to 75 miles or so each. This made for better voice circuits to the Dispatcher, since the physical wire sections were short, and the carrier overhead made the use of voice repeaters unnecessary. We had our own microwave system on the main line from Denver to Grand Jct. to Salt Lake that also helped with this, and in later years this was used to link VHF radio base stations all over the line to Denver so the DS could talk directly with the train crews via radio.