Mobile Exchanges



Mobile Exchanges
by John Chenery

Over the years, a variety of mobile exchanges were built to provide extra exchange capacity where it was most needed and to give emergency cover in the event of a disaster. Typical equipments fitted ranged from special UAX variants, Non-Director to TXE2 as well as containerised Digital and customer switch replacement modules.

Pictured is Mobile Non-Director Exchange No. 341

HISTORY of the Mobile

The very first Mobile Automatic eXchange (MAX) was brought into service on 7th December 1938 by the Postmaster General, Major G.C. Tryon, M.P. who made the inaugural call from Post Office Headquarters to the chairman (Mr. Hugh de Haviland) of Essex County Council in Chelmsford. After the ceremony, the exchange was then put to work in North Weald, Essex where it replaced a small manual exchange.

The early MAXs housed UAX No. 12 type equipment with provision for 100 lines-90 subs. and 10 junctions. The mobile had many special features:-

Trailer Design

Approximate base dimensions 15' 9" by 7' 0."
4-wheel chassis trailer with torsion bar springing on each wheel and Lockheed hydraulic brakes.
total vehicle weight (fully equipped) of less than 6 tons.
front wheels mounted on a turntable for extra manoeuverability of unit in confined spaces.
fabricated, welded steel sheet construction for maximum strength.
steel sheet lining with 1" thickness of cork insulation between shells.
recessed floor (6" below wheel tops) for greater stability during towing.
brakeman's cabin to comply with latest MOT regulations.
brakeman's cabin accessible only via an external lockable door for on-site security.
separate parking-brake lever for use during manual positioning of trailer.
2 horse-power petrol engine coupled to a generator (mounted on anti-vibration fittings) producing 500w d.c.
2 gallon petrol tank with external access for safety.
radiator cooling of engine water.
radiator top-up from roof collected rainwater via underfloor storage tank.
Once on site, the pneumatic tyred wheels would usually be replaced by heavy cast steel feet, bolted directly onto the wheel axles.

Exchange Design

Complete with 2 battery lockers (over wheel arches) each containing twenty five 72 ampere-hour cells of the traction type.
power panel with charging facilities from the generator or a.c. mains via a Tungar rectifier.
one C unit, two As and two Bs.
U/G feed via a flexible steel tube terminated on a special mechanical joint fixed on rear wall.
MAX12s were often used to provide service during the conversion from manual working while the permanent auto exchange was being constructed. Typical setting-up time including diversion of local cabling was 2 weeks. 


MAX13s, with a 200 line multiple, were the mobile version of the UAX13 and needed 2 trailers.


Trailer 1 (Section A)
A units
C units
Trailer 2 (Section B)
B units
Power equipment


The Sixties
Whereas, MAXs were used predominantly for rural areas and to aid the conversion from manual to auto working, the demand for telephone service was increasing and a larger capacity expedient was needed. Two new style mobiles were designed; the MNDX  (a subscribers' unit) and the MTX (a tandem unit).  In the early Sixties, an initial distribution of 30 units was made to the Regions with more on order.
The New Mobiles 
Mobile Non-Director Exchanges (MNDXs) and Mobile Tandem Exchanges (MTXs).

Key features of the new mobiles were:

dimensions 21 feet 8 inches long by 7 feet 6 inches wide by 13 feet high.
2000 type equipment on 8 feet 6 inches high racks. 
standard Non-Director exchange practice.
twin pneumatic tyres, front and rear.
MNDXs for subscriber lines.
MTXs to off-load junction traffic.
The first Mobile Non-Director Exchange (MNDX) called Crompton was brought into use in February 1963 as relief to Shaw auto exchange in Lancashire.

MNDX1 was produced in 1962 by the Post Office Factory at Enfield. A single MNDX could provide service for 400 subscribers. 

Later in 1963, relief to Woking manual exchange was provided by Mayford which used two of the MNDX subscriber units and one Mobile Tandem Exchange (MTX) unit to provide 800 additional lines.

MTXs utilised some of the equipment within the MNDX, so could not be used in isolation. In the late Sixties, MTXs were produced at the Post Office Factory at Birmingham.

Later models of the new mobiles had smaller pneumatic or solid tyres, suitable only for transportation by low-loader.
Mobile on low loader


The Seventies
In the early Seventies the waiting lists for telephone service were growing due to the shortage of switching equipment in some areas. To improve provision, the Post Office placed an order (worth £3 million) with Plessey Telecommunications for 30-40 new mobile exchanges. The TXE2 electronic exchange was chosen to bring relief both to small towns and as a temporary replacement at exchanges undergoing refurbishment.

In 1972 the Post Office had approximately 250 conventional (Strowger) mobiles and 350 additional ones were on order, including 25 MXE2s of 1000 line capacity and 10 of 2000 lines.

The MXE2

The first mobile TXE2 designated MXE2, was operational in 1973 at Padgate, Lancashire to cater for growth in Warrington new town. Many MXE2s were equipped at Plessey's factory in Beeston, Nottingham. 
Key features were:

A trailer 27 feet long (8.2 metres)
A reduced rack height of 8 feet 11/2 inches instead of the normal 10 feet 6 inches.
1000 lines contained within two trailers, a line and switch trailer and a control trailer.
Line & switching trailer- line units, A,B,C & D switches, supervisory relay sets, meters, distribution and trunk connection frames.
Control trailer- control equipment, registers, power plant, workbench and test equipment.
10 hour standby provided by batteries.
The first 2000 line MXE2, of contained within three trailers (2 line switching and 1 control trailer) was commissioned at High Wycombe in March 1974. A reduced battery standby time of 5 hours was possible.

Typical set-up time of the MXE2 on-site was approximately 6 weeks.

Logos & Serials




Mobiles were finished in dark green and allocated sequential unit numbers in white lettering together with 'POST OFFICE TELEPHONES' or 'POST OFFICE ENGINEERING DEPT.' as well as, the superfluous 'TELEPHONE MANAGER' as many mobiles were used nationally and not assigned to particular Areas. The GPO logo complete with crown was also used for some time after the Post Office ceased to be a government department.
MNDX1 carried trailer serial number  T11892

Telecoms trailer serial numbers T205000-T207418 were used between 1965 and 1971. Thereafter, the 9 digit telecom vehicle identification numbers were used. Thus for MNDX No. 331 serial 71 830 0030 the details are as follows:


71 - Year of Delivery
830 -
Mobile telephone exchange
- Unit serial number

1971 is the year of delivery (but not necessarily when the racks were fitted). The 830 refers to mobile telephone exchanges and the 0030 is the unit serial number. 

Between 1971 and 1981, serials 830 to 834 were also used to designate:


Mobile telephone exchange trailers.
Mobile audio station trailers.
Mobile carrier station trailers.
Mobile radio station trailers.


The Eighties
The Eighties saw a steady increase in  the number and capacities of electronic exchanges and the introduction of digital technology, System X. The Strowger mobiles were becoming outdated and unnecessary as smaller more efficient switches were being developed. Strowger was being exported in containers and at home, Commsure was using the spare mobiles to house replacement PABXs. This was a decade of change as long established practices gave way to new methods of  working.

In the early Eighties, British Telecom's Teletrade produced complete Strowger exchanges in insulated, air conditioned standard 20 feet containers for export overseas. The basic unit was 400 lines capacity. Lettering was British Telecom blue on a yellow container.
Containerised 'reed switching systems' were also produced by Plessey for both the home and export markets.

Commsure was devised by Phil Taylor in response to customer demand for an emergency (PABX) switch replacement service that could be used in the event of a fire or other major disaster at a business premises. This was necessary as the Stored Program Control PABXs in use during the Eighties took many months to order and to program to customer's requirements. The idea was adopted by British Telecom North East in 1984 and the first units were ready by February 1985.
Commsure units were:-


suitable for British Telecom maintained switches of 250 extensions upwards.
converted MNDXs type trailers, 7.3 metres long by 2.3 metres wide.
finished in white with a brown and cream banding carrying the 'T' and Commsure name.
guaranteed delivery within 24 hours.
air conditioned.
fitted with a 500 extension SPC PABX, with a capacity for 48 exchange lines and 48 private circuits.

Additionally, each unit was fitted with a standby power supply and a separate operator suite with space for 2 operators and a manager/supervisor. A full set of alpha directories (on fiche) complete with a microfiche reader was also supplied as part of the package.

Once delivered to site and connected to the customer's internal wiring, the units were expected to be loaned for a period of  up to 6 months.

The world's first transportable System X telephone exchange was delivered on 1st April 1987 to BT's East Anglia District as an expedient to cater for the rapid expansion of the Port of Felixstowe. Supplied by Plessey Major Systems Ltd of Liverpool, the RCU (Remote Concentrator Unit) with a capacity of  approx. 1500 lines, was housed in a standard 30 ft container which was positioned by a heavy lift 'Quinto' crane. The exchange keys were handed to EAD's District Manager, Colin Coleman by Plessey's Paul Leidecker.

The Nineties
The Nineties saw the end of the Analogue exchanges with the majority of  the Strowger and Electronic mobiles having been stripped-out for use as storage units, mobile shops and other purposes. Fully-equipped surviving units are rare, but examples can be seen at museums and preserved railways.

Network Mobiles
The digital network is perhaps more prone to a major failure as a result of human error such as a software problem (e.g. the 'Year 2000 bug') than the more resilient Strowger days. Even so, modern equipment is still vulnerable to fire or flood and there is still a need for mobile exchanges as we continue to rely upon the telephone service as a vital communication aid in today's global village.
To this end, the concept of mobiles still have a place as containerised digital node switches which have been produced to replace at short notice any trunk switch in the event of a serious failure.

Two Thousand & Beyond
Having reached the new Millennium, mobile trailers could be used for conceivably any purpose ranging from a complete Personal Computer (PC) equipped office, Internet node, mainframe system, microwave link-up and broadcasting, as the various communications medias merge into a single data-stream.

Other mobiles in preservation

MNDX 127

MNDX No.127 
The Telephone Museum, Milton Keynes has MNDX No.127 (T205409) which was first used in Scotland in 1966 and later at Chelmsford, Dalkeith, Hemel Hempstead, Houghton Regis and Kempton Bedford in 1972. It was one of seven due to be scrapped by BT in Bedford during the early Nineties. 

MNDX 295

MNDX No.295 
MNDX No.295 stood for about 15 years at Lochgilphead, Argyll until it was removed circa 1998 to Hack Green Secret Nuclear Bunker, near Nantwich in Cheshire... 

MNDX 323

MNDX No.323 
MNDX No. 323 was discovered in 2002, languishing in a BT exchange yard near Blackburn where it had stood for 10 years prior to being rescued. It has now been donated to a Midland based preserved railway where a full restoration is planned for 2003/4 after which the exchange will be made part of the existing internal Strowger telephone system. 

MNDX 331

MNDX No.331 
Avoncroft Museum of Buildings has MNDX No. 331 (71 830 0030) a 400 line mobile which was last used in the Reading Area as one of three units with the identity Brookside 8xxxxx. 


MNDX 341

MNDX No.341 
The Teradyne mobile MNDX No.341 of  Martin Loach.
MNDX No. 341 was a one of a batch of 200 mobile exchanges produced in 1972 for the GPO and was used in the South East Region. It was designed to be moved on low-loaders, hence the wheels are only to support the trailer and allow manoeuvring once on site.  


The remaining mobiles exist only as storage vans or have been converted to other uses:

MNDX No. 318  & 71 830 0023, 71 830 0098, 72 830 0051, 72 830 0074  have been seen at Ardrossan, Ayrshire. One was seen painted in 'Business Systems' dark blue.

MNDX 72 830 0063
MNDX 72 830 0063
On the Kent & East Sussex Railway, MNDX, Serial No. 72 830 0063 has been stripped out and fitted with UAX Strowger kit... 



Telephony by Atkinson; Chris Barlow; Mike Fletcher; Martin Loach; Peter Walker; Post Office Telecommunications Journals; Telephone Museum, Milton Keynes, & Phil Goodwin; Kent & East Sussex Railway, & Nick Wellington; Post Office Vehicle Club; THG; Teletalk, staff newspaper of EAD.


POEEJ Vol 31 October 1938 -Inauguration of the First Mobile Unit Automatic Exchange.
POEEJ Vol 32 April 1939 - A Mobile Automatic Telephone Exchange by R.W. Palmer and G.A.O. Abbott
POEEJ Vol 42 July 1949 - A 200-Line Mobile Automatic Exchange by E. Siddall and A.A. Page.

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