Distributed Power Unit (DPU)

Many BNSF freight trains that I have watched had locomotives at the rear as well as at the front. Some trains even have locomotives in the middle. A remotely controlled locomotive is called a Distributed Power Unit or DPU. DPUs allow pulling longer trains with more horsepower without putting too much stress on the car couplers.

Someone asked on Facebook how a DPU (Distributed Power Unit) works. Some of the comments were particularly informative:

Jason Smith Here's what EMD says about distributed power:
Theory of Operation
Integrated Distributed Power (IDP) allows locomotives to be distributed throughout the train, which allows more locomotives to pull longer trains without separation from overpowering. IDP allows locomotives not connected directly in consist to be controlled by the lead-controlling unit.
IDP enables the control of remote locomotive consists via radio messages, which emulate the normal 27 pin trainline connection between the locomotives.
The operator can control the remote consists in two modes: synchronous and independent. While operating in synchronous mode, the remote consists mimic the lead consist for throttle or dynamic brake requests. While operating in independent mode, the remote consists are controlled separately from the lead consist for better control over varying track profiles.
EMD LSI Integrated Distributed Power system is designed to provide synchronous and independent control to up to six locomotive consists located at points along the train, used in addition to the lead locomotive.
The system enables control of the remote units through command signals transmitted over a radio link from the lead unit. Operation of the system is such that several trains equipped with Integrated Distributed Power (IDP) can operate at the same frequency and within radio range of each other.
Advantages of Integrated Distributed Power
• Initial charging time of the train’s air brake system is significantly reduced;
• Brake applications and releases are quicker than in conventional consists, resulting in a shorter stopping distance (up to 30% faster);
• Power efficiency and overall tonnage capabilities are enhanced through synchronous and independent control of the motive power at multiple locations;
• More rapid acceleration and deceleration of the train is possible;
• IDP provides improved control of slack action, which reduces damage to cars, couplings and lading;
• IDP enables long trains to achieve and maintain higher speeds for faster, smoother runs and improved scheduling


Jason Smith Here's an old notice from work showing how BNSF determines dp usage on trains in certain subdivisions: February 18, 2010 ALL OPERATIONS MANAGEMENT MGMT. INSTRUCTION No. 130 TO ALL CONCERNED, SUBJECT: DP Power and Placement Requirements - Manifest Trains Below are Distributed Power locomotive requirements and placement guidelines based on total train tonnage and grades. Minimum DP remote power requirements applicable to DP manifest trains operating on the: Lampasas, Phoenix, Seligman, Thayer North, and Wichita Falls subdivisions Trailing Tonnage | Minimum DP Power Requirement and Placement 8,450 - 11,000 tons | 1 DP remote locomotive (8 RPA) on rear or cut in. 11,001 - 13,000 tons | 2 DP remote locomotives (16 RPA total) either as a single DP remote consist of 16 RPA or as two separate DP remote consists with one locomotive (8 RPA) each. 13,001 - 15,000 tons | 3 DP remote locomotives (24 RPA total) either cut in as a single DP remote consist or separated as two DP remote consists. Minimum DP remote power requirements applicable to DP manifest trains operating on the: Birmingham, Ft Worth, Ft Scott, Galveston and Thayer South Subdivisions Trailing Tonnage | Minimum DP Power Requirement and Placement 10,000 - 15,000 tons | 1 DP remote locomotive (8 RPA) on rear of cut in. 15,001 - 17,000 tons | 2 DP remote locomotives (16 RPA total) either as a single DP remote consist of 16 RPA or as two separate DP remote consists with one locomotive (8 RPA) each. 17,001 - 19,000 tons | 3 DP remote locomotives (24 RPA total) either cut in as a single DP remote consist or separated as two DP remote consists.

Jason Smith RPA in the above means rated power axle. For example, a GP30 has 4 RPA while an ES44C4 has 8 RPA.