I came across a fascinating paper by A. E. Noel, Jr. , in the Bell System Technical Journal: it gives an amazing insight into the coming of the "digital" age in telephone switching, and into many "what if" technologies that never made it off the drawing board.
It's titled "An Experimental Switching System Using New Electronic Techniques"
This is the first of a series of articles describing an experimental electronic telephone switching system employing a number of new techniques. These include use of a stored program, a network employing gas tube crosspoints, time-division common control, and large-capacity barrier grid tube and photographic storage systems.
The article is at