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[quote user="Murphy Siding"][quote user="Bucyrus"]  but with a dedicated train-set, one of the other major advantages is that there is no slack.  Slack is a product of the automatic coupler with its operational tolerance, and there would be no automatic couplers in a dedicated train-set.   [/quote]  ???  No slack?  How would you start a heavy train? I always understood, that slack helped the locomotive start pulling a train one car at a time.[quote user="Bucyrus"] Moreover, because dedicated train-sets would likely be shorter than average loose-car trains, all the cars in the train-set would be structurally lighter than a loose-car[/quote]      Why would it be shorter?[/quote]Slack can be used to start a train, but from what I understand, it is usually not essential.  If I am not mistaken, taking advantage of slack to start a train was more common and more necessary pre-diesel, and in earlier eras.  However, this was not the purpose of slack.  Slack is unavoidable as being necessary clearance for the moving parts of the coupler, whether automatic or link and pin (unless you have a coupler system that takes out the slack after a joint has been made). And because slack runs in and out, it can accumulate force.  To prevent coupler breakage from slack surge, the draft gear is sprung.  The sprung movement of the draft gear adds to the potential movement of slack.  So my assumption is that with permanent, slack-free couplers, you don't need cushioned draft gear.I think I should retract what I said about dedicated train-sets being likely to be relatively short.  I was mainly thinking of minimizing the amount of capital that would be tied up say for a repair to just one car in a dedicated train-set.  But there are so many other factors that come into play that I don't see any one overriding reason to keep the consist short.  It may very well be applicable with economic advantage to relatively long unit coal trains for example.  However, in cases where a dedicated train-set is shorter than the general maximum length trains of today, all of the cars of the dedicated set can be structually lighter than the loose-cars that must be able to stand being toward the front of a long conventional train and subject to the slack in that long train.  So that is an incentive to make dedicated train-sets relatively short.
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