In rail transport, distributed power refers to the physical distribution—at intermediate points throughout the length of a train—of separate motive power groups. Such 'groups' may be single units or multiple consists  and are remotely-controlled from the leading locomotive. The concept of the distribution of motive-power was originally developed to permit the operation of longer trains where operational considerations or economics required it, however distributed power has since also been used under circumstances where it is desired to have motive-power at each end of a train simply for reasons of operational flexibility.
Advantages and disadvantages
The greatest benefit of distributed power—and the reason for development of the original concept—is the reduction of drawgear draft forces, permitting the doubling in the size of trains without exceeding draw-gear strength, through the use of mid-train locomotives.There are also potential train handling benefits; over an undulating track profile, a skillful engineer can manipulate the relative power outputs (as well as dynamic and air brake applications) to minimize run-in and run-out of coupler slack throughout the train."
Reduced draft forces along a train will reduce the lateral force between wheel and rail on curves, thus reducing fuel consumption and wear on various running-gear components as well as the potential for a 'stringline' derailment.
Another benefit is quicker application of standard air brakes. With all braking control at the front on a conventional train, it can take several seconds for brake-pipe pressure changes initiated by the engineer to propagate to the rear. Under radio-controlled distributed power operation, the brakes are set at remote locomotives simultaneously with the command initiated on the lead locomotive, providing a more uniform air brake response throughout the train.
The main disadvantage is the operational time needed, and track configuration required, to add and remove the mid-train locomotive consists. Secondary disadvantages are the costs associated with equipping locomotives with the extra control apparatus and the potential for the intermittent loss of the telemetry signal. This latter is known as 'Communication Interrupt' and is coped-with by fail-safe software controls.
Since the 1960s, railroad distributed power technology has been dominated by one company, Harris Controls (originally Harris Corporation — Controls and Composition Division, later purchased by General Electric, and now known as GE Transportation Systems Global Signalling), who have manufactured and marketed a patented radio-control system known as Locotrol that is the predominant (perhaps only?) wireless distributed power system in use today around the world.
With its origins in the early days of SCADA technology for the remote control of pipelines and electric utilities, and from an early concept of Southern Railway President D.W. Brosnan, Locotrol was a product of the North Electric Company (Galion, Ohio) which was later purchased by Radiation Inc. (Melbourne, Florida) and—in turn—purchased by Harris Corporation (also headquartered in Melbourne, FL), and was first tested on the Southern Railway in 1963. The first production Locotrol was installed on the Southern Railway in 1965.
In the early years of this technology, Wabco also had—for a relatively brief period—a competing system called 'RMU' (Remote Multiple Uniter) which was installed on a few North American railroads. However this system did not prevail and soon went out of production. Prior to the advent by North Electric of the proprietary 'LOCOTROL' name, the product was referred to as 'RCE' (Radio Controlled Equipment) or 'RCS' (Radio Control System) and the lead and remote units as 'master' and 'slave'. The colloquial 'master' and 'slave' terms, though, were not formally used by the manufacturer. In some U.S. railroad parlance, Locotrol trains are referred to as 'radio trains'.
A locomotive that has been fitted with Locotrol DP equipment may be set up as either a Lead or Remote 'active' unit; the Lead unit being the controlling locomotive. Only one distributed power-equipped locomotive in any Lead or Remote consist (group) is active. Other locomotives MU-coupled to this 'active' unit operate conventionally as multiple units.
There are two basic modes for over-the-road distributed power operation. Locomotive control can be synchronous (MU), whereby control commands made by the engineer in the Lead unit are transmitted instantly via radio telemetry to—and are followed immediately by—all Remote units in the train, or independent whereby the engineer may set up and independently operate the Remote locomotives as a 'front' and a 'back' group (or with Locotrol III and subsequent versions; as 'Lead', 'Remote-forward', Remote-intermediate', 'Remote-rear', and 'Remote-trail' groups—this last at the rear of the train). The front group always includes the Lead locomotive, and all Remote locomotives in the front group follow the commands made by the engineer using the Lead locomotive controls. Which Remote locomotives are in the front or back groups are selectable by the engineer in real time. One DP train cannot affect another DP train or another individual DP-equipped locomotive not in a train; and an individual DP-equipped locomotive not in a train cannot affect any DP train or other individual DP locomotive regardless of proximity.
Distributed power was originally able to be provided at only one intermediate location within a train. These forerunner systems (Locotrol 102-105 and Locotrol II) required a radio-relay car to be attached via standard multiple-unit jumper cabling to the remote locomotive(s) to provide the radio-control commands and facilitate feedback signals. Later, Locotrol II evolved into the 'Universal' system in which the radio-control equipment was installed on the locomotives themselves, rendering the relay car (variously referred-to as an 'RCU' for remote control unit or 'LRC' for locomotive remote control) redundant.
Locotrol III was the next development—being compatible with both the Knorr-Bremse / New York Air Brake CCB[disambiguation needed] and Wabtec's EPIC electronic locomotive brake equipment, and permitting multiple Remote unit locations as described above. The latest incarnation of this equipment is Locotrol Electronic Brake (LEB), which integrates the GE Locotrol technology with K-B/NYAB's CCBII brake .
Distributed power is used in the United States and Canada, China, Australia (Queensland, the Pilbara region of Western Australia, and in the southwest of Western Australia), Brazil, Germany, and South Africa. It is also (or has been) in regular unit-train operation in India, Mauritania, and Mexico, and almost made it into operation in both pre- and post-revolutionary Iran.
In the south of Western Australia, Locotrol is used in the 'top-and-tail' configuration rather than specifically for long-train operation. With the recent advent of electronically controlled pneumatic brakes (ECP)—either hard-wired or radio-controlled—and integrated electronics for locomotive control and engineer's cab display systems, DP can now be provided via the ECP brake communication media, and other manufacturers are able to provide this capability.
A recent DP system from Wabtec, called PowerLink (which can be either wired or wireless) is in use in Queensland on narrow-gauge coal trains and in the south of Western Australia on standard-gauge iron ore trains.
Other similar operations
'Top and tail' is a phrase used to describe an operation where there is a locomotive at each end of the train; usually to make it easier to change direction at a terminal location where it is not possible to run the motive power 'around' the train (i.e. swap the locomotives from one end of the train to the other); this arrangement is not used specifically to operate longer or heavier trains.
The description should not be confused with 'push-pull', which refers specifically to a train configuration in which the motive power is located at one end of the train only. In this latter configuration, the train is able to be operated from the 'non-powered' end by use of an engineer's control position (the 'cab-car') located at that end of the train. Push-pull operation is usually associated with passenger trains.
'Top-and-Tail' is not, strictly speaking, a Distributed Power operation although such a configuration could, conceivably, be used as such. The remote control of a Top-and-Tail configuration is mentioned above (Western Australia) in which Locotrol technology is utilised to provide a solution although not in the context of increasing the train size. The term 'Distributed Power' has been specifically coined to convey the generic concept of removing some of the motive power of a train from its head-end and distributing it within or throughout the length of the train to reduce coupler forces (and, when competently operated; in-train dynamics) and permit the operation of longer, heavier trains. Use of this term ensures that the brand-name 'Locotrol', which is the property of one company, is not itself used generically to refer to the technology of distributed power.
- "Railways in the Coal Fields of Queensland". Technology in Australia 1788-1988. www.austehc.unimelb.edu.au. p. Chapter 6, page 382. Retrieved 2015-02-15.
- Lustig, David. (September 2010). "Freight Train, Unbounded: Distributed power: It's a bigger deal than you think". Trains Magazine (Kalmbach) 70 (9).
- RailwaysAfrica 2008/4