How Do Multiple Locomotives Work Together?

As a lifelong rail enthusiast, I have often pondered about the mechanics behind multiple unit operation and how one engineer can run multiple locomotives at once.

So, how do multiple locomotives work together? Multiple locomotives work together using a series of cables between the locomotives that provide an electric current, keeping the trailing locomotives in sync with the leader. This is performed by a 27-pin connector, that attaches between locomotives in the consist, as well as air hoses controlling the braking system. 

Railways throughout the world operate this way and have found it effective to include multiple units in a consist to increase pulling power, while only needing to utilize one crew. This was one of the selling points for diesel-electrics when they first emerged onto the market.

Multiple Unit Locomotive Operation

Multiple locomotives are operated through the use of a 27-pin connector that supplies electrical current to the trailing locomotives. This operation allows the circuits of the lead locomotive to control the circuits of the trailing units. In addition to the 27-pin connector, locomotives also have certain air hoses that control the independent brake, the independent actuator, and the main air reservoir. When the locomotives are assembled and multiple unit hoses attached, the locomotives in the consist form a large air reservoir that allows extra air pressure to bring heavy trains to a stop. Additionally, the brake pipe, which is the air hose located closest to the coupler, provides air pressure to the entire train in order to activate the brakes on each car. In the cab, the engineer prepares the consist by programming the first locomotive as the leader, and the rest as trailing units. The crew then checks to ensure that every unit is online and that the brakes are working effectively.

Bill Johnson

Who Developed Multiple Unit Operation, and Where Was it First Used?

Multiple unit operation was engineered by inventor Frank Sprague, who is known as the “Father of Electric Traction”. Multiple unit operations were first utilized on the South Side Elevated Line in 1897, which is now the well known Chicago Elevated or “Chicago L”. Sprague’s multiple unit system used pneumatic operation through air pressure, however, although modern multiple unit systems still use pneumatic systems, electrical cables are now used for controlling the prime mover, lights, etc.

A mother-slug set is another multiple unit operation. The slug cannot supply power on its own as it lacks a prime mover, it must be attached to the mother unit which supplies the power. The purpose of the slug is to increase tractive effort, as the diesel locomotive oftentimes produces much more electricity than the traction motors can handle, resulting in wheel slip. The use of the slug gives the locomotive additional traction motors to more effectively distribute the electricity generated.

There are different types of slugs for different operations, yard slugs are low profile to increase visibility while switching cars. A road slug is commonly seen on local trains in the locomotive consist. These types of slugs usually have cabs and dynamic brakes for mainline use. It is common for road slugs to be rebuilt from locomotives that were formerly powered with a prime mover. To maintain weight in lieu of the prime mover, the fuel tank is usually filled with concrete, which increases the tractive effort of the mother-slug pair.

Jeff Hampton

Another type of multiple unit train is when the entire consist is self propelled. These trains operate either by electric traction, or are powered by a diesel prime mover. The electricity gathered from overhead wires or generated by the diesel engine is then sent to the traction motors on each car. These types of trains are similar to the multiple unit capabilities of a locomotive, however, the power generated is transferred throughout the entire train, instead of just the locomotive.

Many of these types of multiple units operate as a married pair, and is semi-permanently coupled to the car that houses either the diesel engine or the pantograph.

An example of this would be the Metro North M8 train sets, where the electric current is either gathered by a third rail or the pantograph, and is distributed throughout the entire train set. Another example are the new Eurostar train sets, the e320s, which are set up as electric multiple units (EMUS). Additionally, most subway systems around the world use this type of traction for power as well, as it is more economical for use underground.

Most multiple units are used in passenger operation, as they are economical and easy to turn around at the end of a run. However, some freight carriers throughout the world use these train sets in their daily operations as well.

Worldwide Use of Multiple Units

In North America, most locomotives are compatible with each other due to a universally implemented Association of American Railroads (AAR) system. This is due to the fact that most freight trains in North America operate with at least two locomotives in the consist.

This was not always the case however, as during the early days of dieselization, locomotives from different manufacturers were not compatible. Additionally, many early diesels were not delivered with multiple unit capability, and was added later.

In the United Kingdom, a similar practice was adopted, however, some locomotives are not compatible with others. This is differentiated by a blue star, a red or green circle, an orange square, red diamond, white diamond, and yellow triangle. This symbol is located on the front of the locomotive, close to the multiple unit cables. For example, if a locomotive has a blue star, it is only compatible with another locomotive that displays a blue star. So a locomotive with a blue star, and one with a red circle could not work together, and are not compatible multiple units.

Note the blue stars below the marker lights on this Class 37.
Matt Spencer

For further information and pictures of modern diesel locomotives, Brian Solomon’s publication, “Diesel Locomotives” takes a detailed look at the modern locomotive. Solomon’s publication is available here. (link to Amazon)

Related Questions

Are steam engines capable of multiple unit operation? In the days of steam, double heading was not as common a practice as it is today. Double heading steam locomotives only occurred when a steep mountain grade must be traversed. Steam locomotives did not have multiple unit capabilities, as there would have to be an engineer, fireman, and conductor on each locomotive in the consist. This was an expensive ordeal, as more than one crew was needed in order to operate the train.

What is a pusher locomotive? A pusher locomotive pushes the train from the rear, usually to assist in climbing steep grades. The benefits of a pusher locomotive is the ability to add extra power to the rear of the train while reducing slack action. A similar practice is the use of a distributed power unit (DPU) on the rear or in the middle of the train. The benefits of the DPU include the ability to add extra power to the train without the need for an extra crew, and gives it the ability to haul longer trains. The DPU not only adds extra power but also reduces slack, and as a result, reduces the risk of a knuckle coupler breaking.

A basic knuckle coupler is allocated to withstand about 250,000 lbs, with some more heavy duty couplers, able to withstand 400,000 lbs. With the addition of a DPU locomotive, if the locomotive is putting too much force on the knuckles, the DPU will alleviate the burden, and reduce the amount of slack significantly, reducing the chances for a broken coupler.

When multiple locomotives are in a consist, why are some facing the opposite direction? Locomotives face the opposite direction so it is easier to turn the train around when they get to their final destination. Instead of the need to spin the power completely, the crew just has to flip a few switches, and change the designation of the new leader from trail to lead, and the other locomotive from lead to trail. No matter what way a locomotive is facing, pulling power is similar in either direction.

What is a locomotive consist? A locomotive consist is how many or what type of locomotives make up a train. An example of a locomotive consist could be a number of diesel-electric locomotives coupled together to provide the power to a train, as it is called a consist of locomotives.  




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