Exactly - see http://www.8052.com/forumchat/read.phtml?id=148737

In addition to the "line driver" of the original post (which just launches the signal onto the long wires), he will also need a suitable modulation scheme - which is precisely what a modem does.

Remember that PSTN modems achieve speeds well over 9600bps on lines that may be several miles long and over a hundred years old!
And these same wires are used by digital technologies like ISDN and ADSL...

So, again, look at how the telephone system does it - forget the bits you don't need (ringing, an exchange, etc), and just look at the line drivers.
In particular, look at "private wire" circuits...