US Locomotive MU Control

Introduction to Multiple Unit Control on US Diesel Electric Locomotives

This is an edited description of the standard AAR (Association of American Railroads) arrangements for multiple locomotive operation in North America originally prepared by Jonathan Hollahan.  Much additional information on locomotive operation is included. 


There is one standard arrangement approved by the AAR and its member railroads and manufacturers to enable each unit of a locomotive consist to function as one from the controls of any unit in the consist.  Note that I am not speaking of the "Locotrol" or other radio apparatus for remote control; merely about more than one locomotive coupled together and controlled from one cab, in North American practice.  There have been other schemes and variations in the past, such as the Baldwin pneumatic system, but these are NOT compatible with the existing standard and cannot be used in AAR interchange service.  Some locomotives, generally older, low-hp switchers, are not equipped with any mu (multiple unit) system.

Connections Between Locomotives

There are two basic connections required between locomotives to achieve the mu operation. One is the independent (loco-only) air brake and the second is all the other functions. The air brakes of the locomotives are controlled by the three (or sometimes, four) additional air hoses mounted on both pilots (headstocks) of each loco on one or both sides of the coupler. All other functions, whether mechanical or electrical, are electrically controlled through the jumper cable between units, which is the thick, hose-like cable hung below the walkway plates which bridge the gap between units. This cable contains 27 wires and has large, cast aluminium plugs each end which fit mating sockets on each end of each locomotive. [See "Model Railroader's" Diesel Locomotives Cyclopedia Vol. 2, p. 22-23 for a picture and diagram of these hoses and cables.] Sometimes these cables are removed when not in use and stored inside the loco and sometimes dummy plugs are used to secure the cable on the end of the loco.


The air brakes on a Diesel or electric locomotive are "independent" of the brakes used on the rest of the train. When a train ("automatic") brake application is made, the locomotive will react the same as any boxcar, applying its brakes. However, the engineer can override, or "bail-off" application on the locomotive.

The 3 hoses that run between the units enable the control of all units as one. The hoses are, from the coupler out, MR (main reservoir) ACT (actuating) and BC (brake cylinder), or sometimes AR (application and release). The MR hose couples all of the main reservoirs in the units together, creating one large reservoir to increase air capacity and share the load among the air compressors of each unit. The ACT hose is pressurised when the engineer bails off, to release the brakes on all units. The BC hose is pressurised according to the degree of braking effort the engineer has selected. However, there is no direct connection between the brake cylinders on the different units.

The amount of pressure required for different types of locomotive brake systems (usually 45 or 70 psi) is proportionally controlled by a relay air valve on each unit, which senses the input pressure and sends the proper pressure to the cylinders on its unit. These valves also provide a check-valve function, to trap some air on each unit in case of separation while running. If there is a fourth hose on each side, it is used to mu older locomotives that do not have electrically controlled sanders.

To "hook up" or mu the brakes on more than one unit, only the three (four on older units) hoses, plus the main train line hose under the coupler, need to be hooked up, and the corresponding valves behind the pilot opened.

In the cab, we will assume the units are equipped with 26L brakes, as most are today. In the lead unit, everything will stay the same regarding the air brakes. In a trailing unit, you have to select whether you want the brakes controlled from the unit you are in, or from another unit. The MU-2A valve is located on the side of the control stand below the feed valve. It can be either a knob or a lever. In the "Lead or Dead" position, the independent brakes are controlled from this unit. In "Trail" position, the independent brakes are controlled through the mu hoses from another unit. Failure to position this valve properly can result in sticking or inoperative locomotive brakes.

The feed valve supplies and regulates the air supplied by the automatic brake valve to the train line (brake pipe). It must be isolated from the train line on trailing units to keep from causing an inadvertent automatic brake release. This is done by turning the "cut-out" or "double-heading" cock on the front of the automatic brake valve to "Out" on all trailing units. The automatic brake handle is placed in the "Handle Off" position and the independent brake is placed in the "Off" position. On most units, the brake handles may be removed to prevent tampering. If the handles are left in place, an emergency brake application may be made from the trailing units, regardless of how they are set-up (handy on helper units, for example). Special valves are used to retain enough air to apply the locomotive brakes on each unit should there be an inadvertent separation of the units.

Electrical Connections

The above covers the brakes, now on to everything else. All other functions of the locomotive are conveyed electrically through the 27-wire jumper cable between the units. The cable is about 6 feet long and weighs about 40 pounds (18 kg). It fits into a receptacle usually located just below the walkway on the pilot of the locomotive and is clipped to the underside of the between-unit gangway when in use, to keep it from fouling the coupler. The receptacle has a spring-loaded cover to keep out dirt and water and when the cable is inserted, the cover has a key on the underside that keeps the cable in place in any event short of a separation. All mu circuits are +74 v DC and are all on-off except for power reduction (pin 1) and dynamic brakes excitation (pin 24), which are 0-74 V DC.

The wires within the jumper cable are designated as follows:

1.. Power Reduction

2.. Alarm Bell

3.. "D" Governor Solenoid

4.. Negative Voltage Common

5.. Emergency Sand

6.. Generator (Main Alternator) Field

7.. "C" Governor Solenoid

8.. Reverse

9.. Forward

10. Wheel Slip Indicator

11. Spare

12. "B" Governor Solenoid

13. Control Circuits and Fuel Pump

14. Spare

15. "A" Governor Solenoid

16. Engine Run

17. Dynamic Brake Control

18. Spare

19. Spare

20. Dynamic Brake Warning

21. Dynamic Brake Interlock

22. Air Compressor Control

23. Manual Sand

24. Dynamic Brake Excitation

25. Headlight

26. Ground Relay Reset

27. Spare

Note: On the front of the unit, the 8 and 9 pins have their functions reversed. This, coupled with the fact that these wires are also reversed in the jumper cable, is how a unit knows which direction to run in, regardless of which way it is facing in relation to the other units.

The throttle position and thus the power requested by the engineer, is interpreted for the engine by the governor. The Woodward governor takes the desired power output and changes it into a fuel setting and an alternator excitation setting and constantly balances the two to keep the engine at the desired power output. The signal from the throttle is sent as a 'code' relating to the 5 solenoids the governor uses to control itself. The solenoid pattern for the different notches is:

Low Idle, D, E

Idle, Notch 1, All off

Notch 2: A

Notch 3: C

Notch 4: A, C

Notch 5: B, C, and D

Notch 6: A, B, C, and D

Notch 7: B, C

Notch 8: A, B, C

Stop: D

Thus, the power level requested by the throttle position is mu'ed among the units as a +74 V DC signal on the appropriate combination of solenoid circuits, plus 74 v DC on wire 6 to enable alternator excitation, on wire 16 to power the governor, and wire 13 to enable all control circuits.

A brief explanation of the other circuits:

1. Power Reduction. An option available to reduce the power output of the consist for a given throttle position. Used when traction is bad, or when trying to maintain an exact speed.

2. Alarm Bell. This alarm rings in all units when a damaging condition is detected. The fault is shown by indicator light or computer screen only on the affected unit. Turning the isolation switch in the cab of the affected unit will silence the alarm bell, except in case of a hot engine.

5. Emergency Sand. When the train brakes go into emergency, the sanders are actuated in both directions on all units.

10. Wheel Slip Indicator. While under normal conditions the engineer isn't even aware or concerned about the automatic regulation of wheel slip, if it becomes too much for the circuits to handle this light indicates the need for the engineer to reduce the throttle or take other steps. It can also indicate a locked-up wheelset.

13. Control and Fuel Pump. By controlling these functions from the lead cab, the ability to shut them off in case of fire is maintained.

16. Engine Run. This provides the power to actuate the governor solenoids.

17. Dynamic Brake Control. Actuates the switchgear to change the unit from power to dynamic braking.

20. Dynamic Brake Warning. While the dynamic brakes are normally limited to the proper value for each unit, this light warns of a control failure and the need to reduce dynamic braking effort.

21. Dynamic Brake Interlock. Prevents operation of the dynamic brake if the switchgear is not properly set-up.

22. Air Compressor Control. The air compressors on each unit are made to act as one to share the load and increase the capacity of the air system.

23. Manual Sand. The engineer can activate this at will, and each unit will apply sand on all axles in the direction of travel.

24. Dynamic Brake Excitation. A variable voltage that controls the excitation of the main alternator to control the degree of braking effort.

26. Ground Relay Reset. The ground fault relay is a protective device that guards against shorts in the traction motors and alternator wiring.

Note that even though a unit may not be equipped with dynamic brakes, it still has the wires to pass the signals to other units in the consist.

Setting Up

In the cab, the controls must be properly set for either a leading or trailing unit. In the lead unit, the only thing that must be changed is the position of the headlight control switch, to turn off the headlight facing the other locomotive and send the signal through the mu cable instead. The control and fuel pump breaker must be on. In the trailing units, the engine run, generator field, and control and fp breakers must be off and the headlight switch must be positioned correctly, indicating if the lights are to remain off in a middle unit, or on on the last unit. The reverser handle must be removed from the control stand, or the unit will do weird things when you try to move them, as they do not know which unit to take the signal from.

Once the jumper cables are hooked up, air hoses cut in, and controls set, you must perform a Power Brake test on the consist. First, the independent brakes are released and release verified on all units. Then an automatic brake reduction is made and application is verified on all units. Then, with the automatic brake still applied, the independent brake is "bailed off" to release the application on the locomotive only and the release of all shoes is verified. Then the brakes are placed in emergency and the application of all brakes is verified, along with proper function of the sanders.

With the brakes applied the throttle is opened to verify that all units are responding to the throttle and each unit is checked to make sure that it is producing roughly the right power. With the throttle still open, the brakes are again put into emergency and the dropping of all units to idle-no power is verified.

© The Railway Technical Website 2019