The Mobile Automatic eXchange was (and still is) a very useful and valuable resource. At the time when UAX's where being introduced the vast majority of rural exchanges were of small capacity (many were well under 100 or even 50 lines), and quite probably operated during the day time only. They were also quite likely to be located in a room of a private house, and looked after by 'caretaker' operators.
Very often it was found that these rural exchanges would need urgent replacement, for various reasons including: Death of the caretaker-operator; or replacement of small manual or automatic exchange due to fire or flood. When it is considered that trying to find a suitable site for a permanent building to house automatic equipment may take up to a year, a temporary exchange that can just be wheeled into position has many advantages. Indeed such a unit could also be used to allow the replacement of an existing exchange with a larger one in the same building or to expand an existing exchange which has suddenly become exhausted.
The normal provision of a temporary exchange would require finding a site, erecting a building, delivery of the equipment, cabling of the units and power plant, and then testing it all out. If an exchange could arrive ready to go in the back of a trailer, then the only work required would be to find somewhere to park the trailer, modification of the external plant, and perhaps the modification of the subscribers telephones; and all this work could be done while the mobile unit makes it’s way to site.