Editing Distributed power (section)

==Technology==
Distributed power should not be confused with multiple-unit operation, which is a capability, generally found on all locomotives, that connects 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 operator in the cab of the lead unit. This is accomplished via a 27-pin MU cable and the connection of three additional air lines separate from the brake pipe (sometimes also called the "train line"). DP is an entirely separate arrangement to control locomotives that are physically separated from the lead unit(s) and therefore impossible to connect via multiple-uniting.

The first locomotive at the head of the train is called the "lead unit", and "remote units" can be located at separate positions throughout the train. Each DP remote unit can be MU-connected to other units at that position to provide one or more "remote consists".

The setup and linking of the DP lead and remote units is fairly straightforward, and the air brakes of the remote units also have to be properly configured 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, it is not uncommon for the link to be temporarily lost.

As long as there is not an emergency or penalty brake application that eliminates the link, the system will reconnect automatically or the operator can attempt to reconnect manually. Originally the loss of connection would result in the remote units remaining in their last-commanded throttle or dynamic braking position. Later system versions and software updates typically reduce the remote unit power setting to Notch&nbsp;4 by default if the link is lost.

Although the DP signals from lead to remote units (and vice versa) are nominally instantaneous, in reality it generally takes at least a few seconds for a remote unit to respond to a signal from the lead and for the change in status to show up on the DP display in the lead locomotive cab.

Distributed power originally could 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 MU jumper cabling to the remote locomotive(s) to provide the radio-control commands and to facilitate feedback signals. Later, Locotrol II evolved into the "Universal" system in which the radio-control equipment could be installed on the locomotives themselves. With this option, the relay car — variously referred to as an RCU (remote control unit) or LRC (locomotive remote control) — was rendered redundant.

Locotrol III was the next development, 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. The latest incarnation of this equipment is LEB (Locotrol Electronic Brake), which integrates the GE Locotrol technology with K-B/NYAB's [[CCBII]] brake.