Mobile Exchanges
by John Chenery
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.

Photo: A 'green caravan' at Amberley Museum © LSA April 2008.
| EXIT | Introduction | MAX | TASS | The New Mobiles | MXE2 | Logos | TXD | Containerisation |
| MUIs | DSLAMs | Preserved Mobiles | London Test Section | Acknowledgements |


The very first Mobile Automatic eXchange (MAX) was brought into service in 1938 by the Postmaster General, Major G.C. Tryon, M.P. who made the inaugural call from Post Office Headquarters to the chairman 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.

MAX - Mobile Automatic Exchange

The early MAXs housed 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 were the mobile version of the UAX13 with a capacity of up to 200 lines.

By 1955, 29 MAX 12s (serials 1-29) had been allocated to the GPO Regions. As well as 11 MAX 13s, (serials 1001-1011). 
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.

TASS Mobiles

In 1954 the Teleprinter Automatic Switching Service (TASS) for the transmission of telegrams across the UK was completed. It served 500 telegraph offices. In 1955 two mobile units were provided as emergency switching allowing for 100 station lines and 70 trunks contained in two trailers. Mobile VF telegraph equipments providing 84 channel circuits in one trailer were also on order.

The New Mobiles

As Subscriber Trunk Dialling (STD) started to be introduced in the 1960s and the demand for telephone service increased dramatically, the MAX 12s and 13s were no longer suited to provide either sufficient lines or code routings.

Enfield  1967
 Two new style mobiles were designed; the MNDX (a subscribers' unit) and the MTX (a tandem unit). 

Photo: A mobile exchange trailer with tow-bar at Enfield factory in 1967 © BT Heritage.
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 up to 400 subscriber lines per trailer.
  • MTXs to off-load junction traffic.
The first Mobile Non-Director Exchange (MNDX1) with an exchange name of Crompton was constructed in 1962 by the Post Office Factory in Enfield. The MNDX was brought into use in February 1963 as relief to Shaw auto exchange in Lancashire. 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.

By 1965, 35 MNDXs (serials 1-35) and 7 MTXs (serials 1-7) had been allocated to the GPO Regions. In 1971 an additional 200 MNDXs (serials 200-399) were on order. Eighteen MNDXs of this batch were constructed by the Post Office Factories, while 182 were constructed by the Regions. An additional 40 MTXs were also on order.

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

Photo: A mobile exchange trailer being moved onto a low-loader.
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.

MXE2 Types

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 and switching trailer consisting of: 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 and Serials


Mobiles were finished in mid-bronze 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:

71 830 0030
Year of Delivery 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 audio station trailers.
  • Mobile carrier station trailers.
  • Mobile radio station trailers.
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-foot 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 two 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 approximately 1500 lines, was housed in a standard 30-foot 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.

Later Histories

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.

By the year 2000, mobile units could be used for conceivably any purpose ranging from a complete Personal Computer (PC) equipped office, Internet node, mainframe system, satellite up-link and broadcast facility, as the various communications medias continued towards convergence.


In 2003, new Mobile Infrastructure Units, housed in custom-built containers and equipped by Fijitsu Telecommunications Europe were the latest in disaster recovery planning by BT. Weighing 14 tonnes, the units had a capacity to support 2000 ADSL (Asymmetric Digital Subscriber Lines) connections.

Mobile DSLAMs

The rollout of fibre broadband (Fibre to the Cabinet) in 2010 introduced complex electronics to new roadside cabinets
as DSLAMs (Digital Subscriber Line Access Multiplexers) were remotely sited from telephone exchanges. To guard against prolonged outages in the event of damage caused by road traffic accidents, mobile DSLAM trailers can be deployed as necessary.

2012 Olympics

At Eton Dorney a disaster recovery container (The Hub) was shipped to the area to provide the additional exchange capacity for communications during the Games.

Preserved Mobiles
Preserved Mobiles
Several mobiles have been preserved and restored as a complete working exchange. Others have been rewired or re-equipped, but for many only the trailer survives. 

Photo: MNDX No.341 © Martin Loach
Build Data  
Build Data: Serial numbers, exchange types and initial allocation of units.

Photo: MAX No.29 © LSA April 2014.
London Test Section  
London Test Section
A collection of photos by the late Dave Fairhurst who, along with other London Test Section colleagues, carried out the commissioning of the Mobile Exchanges, which were built at the GPO factory in Bilton Way, Enfield. 

Photo: Rivenhall mobile © Dave Fairhurst August 1968.

Telephony by Atkinson; Chris Barlow; Mike Fletcher; Martin Loach; Peter Walker; Post Office Telecommunications Journals; Telephone Museum, Milton Keynes & Phil Goodwin; 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.
Engineering Instruction  A3216  Oct 1968 - Mobile Non-Director and Tandem Exchanges.
Design, images and text compiled by © Light-Straw. Page last updated 28th April 2017.

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