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In rail transport, Distributed Power (DP) 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. Distributed Power (invented by General Electric in the United States) allows locomotives to be placed anywhere within a train when standard multiple-unit (MU) operation is impossible.
The system allows up to 10 additional radio-controlled locomotives to be distributed with control signals for power and dynamic brake settings via dedicated radio frequencies. The DP units have their air brake systems set up so that brake pipe reductions affect them just as they do rail cars in the train. The system is highly automated and each DP unit to be added is entered by road number into the control display of the lead locomotive. In the DP units the same thing is done with the lead locomotives road number and once the systems are properly set, the units are "linked" via radio frequency.
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- or rear-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 additional 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 wireless distributed power system in use around the world today.
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'.
Distributed Power should not be confused with Multiple Unit operation, which is a capability generally found on all locomotives owned and operated by railroads that connect multiple locomotives directly together via MU cables and air brake control lines. MU operation in North America is designed so that any two locomotives so-equipped and regardless of age and manufacturer, can be coupled together and operated as a single locomotive by one engineer in the cab of the "lead" unit. This is accomplished via 27-pin MU cable and the connection of three additional air lines separate from the "train line" or "brake pipe". DP is an entirely separate system using radio links to control locomotives separated from the lead unit(s) by at least one rail car and therefore impossible to connect via MU.
Distributed Power is a technology invented and owned by General Electric and was first used on the C44-9W locomotives, and EMD locomotives that are DP-equipped (SD70ACe and newer) use GE's system. The lead locomotive of the train is called the "leader" in DP operation, and another "trailer" units can be located throughout the train. Those 9 units are obviously MU-capable and can be MU-ed with up to 4 additional units each in the same manner the lead locomotives are.
Therefore, its theoretically possible to operate 50 locomotives from the cab of the lead unit of the train, with the 50 separated into 10 sets of 5 locomotives each. However, on the DP control screens, only the road number of the leader and the 9 trailers would be entered and displayed. Normal operations rarely use MU-ed DP "trailers" and typical DP applications will have a single "trailer" in the middle of a train, a single trailer at the end or both.
The setup and linking of the DP lead and trailer units is fairly simple and straightforward and the air brakes of the trailer units also have to be properly set up to enable the system to function correctly. The system automatically chooses an available frequency during the linking process so that other DP trains nearby are unaffected. In a crowded yard or in hilly or mountainous terrain, its not uncommon for the link to be lost or to take some time to establish.
As long as there is not an emergency or penalty brake application that eliminates the link, the system will reconnect automatically or the engineer can attempt to reconnect manually. Originally the loss of connection would result in the trailer units remaining in their last commanded throttle or dynamic braking modes and by intentionally disrupting the link, an engineer looking to have a really good fuel economy score for the trip could have the DP unit at the rear of the train do the majority of the work in a higher power setting than the lead unit(s) would be operated in. Later system version and software updates typically drop the trailer power setting to Notch 4 by default if the link is lost.
Although the DP signals from leader to trailers and vice-versa are "immediate", in reality it generally takes at least a few seconds for a trailer to respond to a signal from the leader and for the change in status to show up on the DP display in the lead locomotive cab.
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 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 North 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.
- "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