Line drivers are also known as short haul modems, and are basically modems designed for operation across dedicated pairs. Some use RS422 (balanced 4w), or RS485 (balanced 2w) for inter-modem communication, some use some other method.
Usable distance and speeds vary, generally you trade speed for distance. They are available as port powered, or externally powered versions.
With RS485, you can run them in a multipoint configuration, either to a multiplexer, or a terminal controller. They are commonly used for industrial controls, linking PLCs or other devices, but you used to see them in lots of other situations, such as travel agents, or library terminals, where there were a lot of terminals multiplexed into a single leased-line modem.
Stanford used to have this box called a Gandalf switch, which fed 4w circuits across the campus, allowing connections to a number of devices, including the campus mainframes, and a central terminal server pool. Before the proliferation of PC's, there used to be terminals in every office, all connected to the Gandalf switch. Some departments (EE. CS. etc) had LAN's with local terminal servers. but some places only had a single terminal for a department administrator.
There were two versions of these line drivers, one was externally powered, and had a thumbwheel switch, which allowed the user to select a specific system to connect to. The other was a port powered unit about the size of a pack of cigarettes, and the user selected the system via a menu once the port was activated. The port powered drivers appear to be RS422 based, and they work just fine when wired back to back. I have used them at 9600bps over a 10kft loop.
Speaking of modems, if you have a pair of modems that can be set to operate without dialtone, you may be able to use them over a limited distance over a dry pair. The configuration can be a bit tricky, since you don't have ring voltage, but it does work. You basically have to set one end to answer mode, go offhook, and no timeout.
I have used a USR Sportster to communicate with an Apple Airport over 150' of dry pair. The Airport was running PicoBSD unix, using the modem port as the console.
