Placement Of DPU Locomotives In A Train - Trains Magazine

caldreamer I am trying to figure where DPU locomotives are placed in a train. Do you use the trailing tonnage, weight no matter how long the train is, and/or both of the above? The 'better' answer to this is 'node management' vs. setup complexity.

caldreamer

I am trying to figure where DPU locomotives are placed in a train. Do you use the trailing tonnage, weight no matter how long the train is, and/or both of the above?

The 'better' answer to this is 'node management' vs. setup complexity.

Of course the power is apportioned in tons per equivalent axle (at the 'ruling' maximum train resistance).  As discussed, with trains of sufficient length or grades/profiles of the wrong character, putting all the power on the head end leads to risk of stringlining, slack run-in and -out, and other issues.  

DPU gets around these by supplying the 'tractive effort' as a combination of pull and push.  You will quickly recognize that this means there will be some point in the intervening consist where the pull from the front exactly balances the push from the rear; there is zero buff or draft at this point and it is called the 'node'.  Note that it can freely move forward or backward in the consist, sometimes very quickly, and you can occasionally hear this as a sharp, fast 'slack action' within part of the train. Why this is important is that the maximum tension in any draft gear forward of the node is then limited to what an equivalent train of that length would be, and the maximum buff force limited to that of an equivalent cut from the node back..

My understanding is that you want some bias of the node toward the rear, with the 'tensile' force set to a reasonable maximum as the vehicle dynamics of a train are better with the couplings stretched than when in buff.  If you have the power distributed within the consist as well as 'top and tail' there is effectively a node in each cut between locomotives, and the buff and draft in each section can be less (for a given number of locomotive units) or the train can be much longer -- this is how the 'world-record' trains were made up and handled, otherwise it'd be Big Liz and the Triplex on steroids.

Of course there are implications for programming and running more complex DPU arrangements, so the added 'advantages' of best distribution may not outweigh the issues involved in setup and running.  If anyone hasn't read Don Oltmann's story about railroading on the ex-PRR in the 2040s (in which the concept of multiple 'blocks' with dedicated power to each factors significantly) you should find and read it now.  It would be a significant gain for a number of actual PSR operating models if DPU could be reliably implemented as Don indicates.

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