There were many types of private line circuits, from dry loops (no battery on them) used for burglar alarm and other signaling services up to the highest bandwith of all, the broadcast television channels used to send programs to affiliated stations across the country.
Telegraph and teletypewriter circuits were very common and vast networks served wire services, newspapers, stock brokers and others. They came in 50-, 80-, 75- and 100-wpms speeds. Some ran with customer-owned teletypewrtiers, some ran with Bell Teletypes.
One particular circuit I was familiar with caused a great deal of annoyance to the New York control bureau when Edinburg, Texas, broke to send. Such channels ran on all kinds of physical, carrier, microwave and coax circuits, and by the time the various relays and other hardware along the way did its thing New York would be several lines into the next message before the "break" signal from Edinburg reached New York.
Another circuit from New York to the West Coast and south to Mexico City often had trouble south of Laredo on the Mexican side. The regulations in Mexico required the channels in Mexico be provided by the government telegraph administion, not the telephone come, and it was not unusual for the telco board in Laredo to have to "blind" Mexico City (make the circuit one-way southbound because so many transients interfering with the U.S.A. part of the circuit).
Higher-bandwidth channels came in various bandwiths, some of the most common being voice (telephone)-grade and broadcast-grade, and others. As other have reported, many, many companies had both local and wide-ranging networks, some of them national or even international.
In general, connecting private line circuits with switched service was prohibited, with exceptions for the military services and "right-of-way companies," firms that had their own right-of-way such as railroads and pipeline companies.
At the peak were the television broadcast channels, which had very high quality and reliability standards and were continually monitored in TOCs (Television Operating Centers) all along the way. Another interesting provision of the TV broadcast channel tariff was that there always had to be a hot spare channel available along the entire route to switch almost instantly in case of failure. This was also the only class of service that provided rebates for loss of service more than either 15 minutes or 30 minutes, I forget which..
It should also be noted that there were differences in intrastate and interstate rates for all private line servicess, interstate being cheaper than intrastate as was customary in those days, so it was advantageous to customers to seek to make intrastate circuits segments of interstate circuits rather than stand-alone intrastate circuits.
Along the lines of some of the other reminisences, I remember the downtown Montgomery Ward store in Oklahoma City had the Automatic Electric sets on an isolated (customer-owned) PBX with only Bell phones in a few locations.
One very large industrial customer on the Texas Gulf Coast (it had to be located there becaust seawater was part of the process or feedstock) told the telco how many thousand of dollars they could save (monthly or yearly) with an isolated customer-owned PBX but went with Bell because of the Bell System's ability to restore service along that hurricane-frequented coast. (This concern also prompted them, in the days of much more rudimentary National Weather Service coverage) to maintain their own network of coastal radars from the Louisiana line to the Mexican border.
Wes Leatherock snipped-for-privacy@aol.com snipped-for-privacy@yahoo.com