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Why sometimes trains have two engines one after the other?

  • Hello everybody!

    My question is maybe stupid but I'm going to ask it.  I added a picture to represent what I mean.  Sometimes, I see trains in my area and there are two or sometimes three engines one after the other.  Why is that?

    Thanks for your help!

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  • This situation occurs when more power is required than one locomotive 'unit' can provide.  The engines are electrically and pneumatically connected so they can all be operated from one control position, by one person.  The effect is to make one large 'locomotive' out of several 'units' -- which is a major reason why they are called units...

    In the days of steam, a separate crew would have been required to run each locomotive (and this is still true of locomotives with no multiple-unit (MU) controls, or consists with incompatible MU systems.

    A related issue concerns midtrain 'slaves' and 'trailing power', engines in the middle or at the end of a train.  These are also controlled from the head end, but via some flavor of radio rather than interengine cables and hoses from the lead locomotive.  The current technology used for this is called "DPU" (for 'distributed power') -- there are a number of older technologies each with its own interesting points and history.

    Note that this is a different situation from pushers and helpers, which are locomotives added temporarily to help a train up a relatively short or steep grade.  These generally have their own crews, and are not considered part of the main power.

    "Good lord, you guys do know how to take the fun out of something."

    - Ed Kapuscinski, RyPN, 10/9/2014

  • Remembering that a railroad track is virtually never quite level, and that often there are significant grades to either climb or to descend, the people assigning head-end power, or head end and other distributed power further back in the consist, are taking into account the trailing tonnage and how much horsepower it will take to get that trailing tonnage along the route from point A to point B.  Ideally, you would assign all the power you need for the job, but no more. 

    You would need power to raise the tonnage up the grades, but also dynamic braking power to retard the effects of gravity on a heavy train descending grades. Suppose for the sake of argument that a train will require 2000 hp to accelerate it to 40 mph and to sustain that speed.  If you add as little as a half-percent grade it will require over three times the horsepower to maintain that speed (or, so I have read/understood in what I took to be an informed article by Al Krug).  So, if you had a single diesel capable of accelerating that tonnage to 40 mph on level track and maintaining it, as soon as it hits a 0.5% grade it will need two more of itself in a hurry.  The people assigning power to the train before it leaves the yard would know that there are several grades in the range between 0.4% and 1%, and that the consist will need more than the single unit to do it.

    In most cases on modern main lines, that means multiple units spread throught the entire consist for maximum control.  When a train is draped over a significant hump because it is long enough, you can break draught gear components, for example, because what's lagging behind the up-grade part of the hill is too heavy for the weakest coupler(s) in the bunch, so you would want some power shoving the tail end of the train up the grade, while the lead units are retarding the part already descending just behind them.  It is more complicated than I understand, but that's sort of the idea behind having power spread, or as Overmod says, distributed, throughout the consist.

    Even if you have all the power you need to get the train over all grades, or down them, but one, it is at that one instance that you would need additional power.  That would be 'met' along the way, added to the consist, and the new greater whole would then begni the task of dealing with the grade in question.

    Some people call the steepest grade the 'ruling grade' in a district.  They are mistaken.  The ruling grade is typically the most challenging grade a typical consist can manage on its own with typical head end or distributed power for the tonnages being moved.  If you have to add power along the route someplace, then it's a helper district.  There's no use assigning more power than you need to either run and burn fuel unnecessarily, or to trail it and leave it unused (costing fuel to tow it), when you don't need to and can add it for the shorter intervals when it's utility will count the most.


  • There is also the issue that traffic is not equal in both directions so engines need to be relocated.
  • ndbprr
    There is also the issue that traffic is not equal in both directions so engines need to be relocated.

    Two extreme examples from two different eras on opposite sides of the Pacific:

    1. Japan, 1954.  Two 4-6-2s on a four-coach local, on a line with no significant grades.  No room in the scheduled traffic for a light locomotive move to balance power.
    2. California, 2008.  Six six-axle diesels pulling seven covered hoppers west between Barstow and Mojave.  The hoppers were probably going to one of the two cement plants near Mojave.  Locos probably going over the pass and back to the bottom of the longer, steeper side of Tehachapi.
    And a 'miniature' version - the genset locomotive.  Three engine-generator packages on a single frame. The load to be moved determines whether one, two or all three are on-line.
  • And one more reason I just.remembered. If the railroad is big enough one shop will do the monthly inspections on a single class of engines necesitating all the engines in that class to visit that shop monthly from all over the system.
  • Its all about train tonnage and ruling grade..A ruling grade could be .5% on a curve or a 1.5% or more climb.

    The extra "power" is needed for maintaining track speed while pulling train tonnage,to overcome the ruling grade(s) and curve resistance.



    Summerset Ry.

    "Stay Alert, Don't get hurt  Safety First!"

  • It also helps with when the train needs to turn in oppisite direction
  • trainlover844
    It also helps with when the train needs to turn in opposite direction

    Yep, no turning of the engines needed. Just move the crew to the forward engine and away you go.

    Ken G Price   My N-Scale Layout

    Digitrax Super Empire Builder Radio System. South Valley Texas Railroad. SVTRR

    N-Scale out west. 1996-1998 or so! UP, SP, Missouri Pacific, C&NW.

  • Lake

    It also helps with when the train needs to turn in opposite direction

    Yep, no turning of the engines needed. Just move the crew to the forward engine and away you go.

    Looking again at the photo in the original post, note the cab positions of the two units.  Assuming (from sun angle) that this is an eastbound, all that has to be done to prepare to take a train westbound is to couple up to the west end of the consist, hang a rear-end device on the final car and call for clearance.  No wyes or balloon loops required.

    This is especially useful if the train is running up a branch with no turning facilities.