Railways and telecoms evolved at the same time in the 1800s and without telecoms services railways would simply not operate. Railway telecoms include voice, data, and radio services for rail operations and business use, and while it is possible for trains to move without signalling, telecoms services are essential for running a railway.
The rail industry can have a reputation for being slow to adopt new technology and ways of working, but in the case of telecoms this is not always the case. Here, we take a look at Signal Transmit Receive And Distribution (STRAD) – an electronic teleprinter exchange ahead of its time when it was introduced on the London Midland Region (LMR) at Crewe 60 years ago.
William Fothergill Cooke and Charles Wheatstone obtained their patent for the electric telegraph on 10 June 1837 with the first telegraph line was tested on 4 July 1837 at Camden Town on the London & Birmingham Railway. Twenty of the railway’s directors attended the demonstration, including Robert Stephenson the company’s engineer. On 25 July, Wheatstone’s latest four-needle instrument was demonstrated to Stephenson and he agreed for a permanent circuit to be provided at the railway’s expense. This is considered to be the first commercial electric telegraph line in the world.

Another electric telegraph system was installed between the Paddington and West Drayton stations of the Great Western Railway. This was initiated by the company’s engineer, Isambard Brunel, who had been introduced to Cooke by Stephenson. The London & Blackwall Railway then constructed a telegraph line in 1840 along its three-mile track. This led to other railways adopting Cooke & Wheatstone’s electric telegraph.
In 1842, Cooke wrote the pamphlet ‘Telegraphic Railways’ recommending ‘block signalling’ in which track was divided into blocks or sections into which only one train may enter, with their movement in and out of each block monitored electrically. So, Cooke is thought to be the first to define an electrical signalling system for railways, and telecoms actually preceded railway signalling for the efficient and safe operation of trains.
Most railways developed telegraph networks, with some carrying commercial traffic. Eventually, outside of rail communication was transformed by telegraph networks in all areas of business and life, and telegrams became a popular means of sending messages. This led to the development of more advanced systems, including teleprinters and punched tape transmission, along with telegraph codes such as the Baudot code.
Railways were always at the front of telecoms development and some of the first switched voice networks were installed by railway companies. Even radio links to trains were trialled over 100 years ago. Outside of rail, Telex networks, which were switched networks of teleprinters similar to a telephone network, became popular for sending telegrams or ‘telexes’, and some regions of British Rail introduced their own teleprinter networks.
Just like sending an email today, a teleprinter message provided a record of what was sent and received, which was essential for the efficient control and management of railway operations. So, on the LMR of British Rail, STRAD was introduced in the 1960s.
STRAD
The LMR STRAD system was designed and installed by Standard Telephones & Cables Ltd at Crewe and was only one of a handful of STRAD systems installed throughout the world, with the others provided for military organisations. The Crewe railway STRAD was initially installed with a capacity of 75 and subsequently enlarged to cater for 90 incoming and outgoing channels.
Message switching
STRAD used a protocol known as message switching, whereby a message was ‘stored and forwarded’ and sent as a single unit. There was no dedicated path between the sender and receiver, and message switching is a ‘connection-less’ orientated protocol.
Traditional telecoms networks used a protocol known as circuit switching, which is a ‘connected’ oriented protocol. With this, a dedicated route is established between the sender and receiver before the whole data message is sent, which is maintained until the message has ended. So, even if no data is being sent a dedicated route exists between the sender and receiver which is not always being used.
Modern networks use Internet Protocol (IP) which is a packet switching technology, and like message switching is a connection-less orientated protocol. Packet switching transfers data in a network in the form of packets. The data is broken into small pieces (called a packet) and at the destination all the packets belonging to the same message have to be reassembled. So, a packet is composed of a payload and various control information.
Like message switching, modern packet switching uses the ‘store and forward’ technique while switching the packets. So, the message switching used by STRAD evolved from circuit switching and it was the precursor to modern IP packet switching.

Electronic switch
The teleprinter exchanges found in other regions of British Rail and public telex networks used electromagnetic step-by-step telephony type switches, but the Crewe STRAD was all electronic. Its magnetic drum storage device, which recorded and stored messages prior to their onward transmission, was the only part of the system which was mechanical.
Electronic telephone exchanges were not introduced until the 1980s and with STRAD opening in 1963 it was very much ahead of its time. There were actually two magnetic drums arranged as main and standby, and this, along with a ferrite core, provided a short-term memory function.
Teleprinter messages were transmitted into the central equipment, where they were classified for priority and transmitted to the required destinations in accordance with a keyboard-entered message code. If the outgoing routes were busy, traffic was stored and retransmitted in the correct order of priority and time of arrival, as soon as the required route became free.
STRAD communicated with teleprinters at 50 Baud, but within the system it could handle messages a lot faster and had to be ‘slowed down’ to use the teleprinters available at the time. These were a mixture of transmit/receive and receive-only machines, with more receive-only used than transmit/receive teleprinters. This provided, for example, the ability of a major signal box at the southern end of the West Coast Main Line (WCML) to send a train reporting message of a late running train to a group of ‘receive only’ teleprinters located all along the WCML. The five-bit Baudot code was used along with a start and two end bits, with the eight bits per character transmitted serially.
The internal switching speed of the system was 50 kilobauds, which allowed the retransmission of a message to commence within milliseconds of its receipt, or for it to be stored if no outgoing route was available. Longer duration message storage was also provided in the form of seven tape boxes, each containing up to 100 feet of 35mm magnetic recording film.
A cubicle in the equipment room housed the tape boxes, which also contained tape drive equipment, reading and writing heads, and called the tape machine. The tape was jointed to form an endless loop, with each box containing up to 100 feet of film. This provided nominal storage of 1.5 million characters and assuming an average message length of 30 words, amounted to the recording of over 8,000 messages if required. Messages were sometimes stored overnight if the receiving telegraph office was not open at night.
Wrapped joints were used throughout the STRAD system, which proved to be extremely reliable, and no faults are ever believed to have been caused by a defective wrapped joint. Much of the equipment was duplicated and if a functional unit became defective it was locked out of service automatically, with a duplicate unit taking over. The duplication was not extended to the channelling equipment, but spare units were held ready so that service could be restored very quickly following a failure of an unduplicated unit. This was not a problem though, and the system failed very rarely.
System supervisor
Like a modern telecoms network, the operation of the STRAD network was managed by a system supervisor control position. This was located in Crewe telegraph office on the floor above the equipment room. The console presented the supervisor with audible alarms, lamp displays, and teleprinters, of all the information required to manage the running of the network, along with the controls to enable interventions if abnormal conditions arose.
For example, the amount of drum storage provided was only sufficient to meet the normal busy hour requirements, as storage was very expensive and limited by the size of the magnetic drum. The amount of storage used was displayed continuously in the form of a ‘thermometer’ type lamp display, and an audible alarm was given when the store occupancy reached a predetermined level. In the event of traffic congestion, the supervisor could relieve a heavily overloaded route by arranging for traffic to be forwarded to an alternative destination, or they could place incoming messages in overflow storage for later transmission.
Power supplies and air conditioning
STRAD operated from a no-break 415 Volt, three phase, 45kVA supply, with the no-break consisting of a motor, alternator, and flywheel on a common shaft. The kinetic energy stored in the flywheel was designed to protect the equipment from voltage surges and outages of the external power supply, and a diesel engine was coupled to the main shaft via a magnetic clutch to maintain the supply when required.

In the equipment room, each cubicle had its own regulated power pack to convert the AC supply into the DC voltages required by the electronic circuitry. The magnetic drums had their own 110 Volt three phase supply from a motor-alternator set. This was to enable the drum speed to be maintained within the very close tolerance of 1500 RPM plus or minus 0.5%.
Similar to modern data centres, 60 years ago the designers of STRAD had to ensure that the maximum working temperature of the equipment was not exceeded. The 45kVA supply and over 300,000 semiconductor devices generated a fair amount of heat, so it was necessary to air-condition the equipment room.
Electromagnetic Interference (EMI) is another modern problem that was also a concern to the STRAD designers, with the electronic circuitry used in STRAD and Crewe being a 25kV AC traction area. STRAD was also located next to an electromechanical Strowger telephone exchange. The STRAD equipment room and doors were therefore lined with copper and appropriate bonding. Many of the internal wiring within STRAD also used coaxial cabling, presumably as another defence against EMI.
STRAD went into service in August 1963 and in a technical paper presented to the IRSE in 1966 it was reported that the system was reliably handling 6.5 million messages per year. This peaked at over 7 million messages per year, before STRAD was replaced with the more modern National Teleprinter Network (NTN) covering all of British Rail in the late 1970s. While STRAD was revolutionary for its time, compared with today’s technology it was extremely crude, but it remains a great example of how railways have always used the latest telecoms technology.
In the 1966 IRSE paper it is recorded that the performance of the maintenance staff was very highly commended. The technicians were drawn from the ordinary railway maintenance ranks, having never had any contact with a system like STRAD in their lives. It must have seemed like a system from the future. They were trained on site by the contractors’ engineers, and in a very short time they took over full responsibility and kept the system running with very few major faults.
This is similar to today and how railway telecoms maintenance and asset management teams very quickly learn how to look after the latest technology, such as IP devices (including voice, CIS, CCTV, and transmission), ribbon fibre, cyber security, and GSM-R. We expect this to continue, and rail will always need to deploy the latest telecoms technologies to provide a safe, efficient and reliable railway.
With thanks to Ted Walley, one of the maintainers of the STRAD system, for his help with this article.