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Continuous Traction Effort vs # of pull-able rail cars

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[quote user="PNWRMNM"] Sam, In general terms power assignment is done by train brief which typically gives tonnage and length limits plus power requirements. I think most roads assign power for manifest (carload) and intermodal on the basis of Horsepower per Ton (HPPT). A preferred carload train may get 3 HPPT, a stack train 2.5-3, and a hot trailer train 4-5 HPPT, all over the same territory. Drag freights and unit coal and grain trains are generally assigned based on published tonnage ratings, which as Balt said are just enough to get the train over the road. Just to make it interesting not all locos are created equal for drag service. DC units have a minimum continuous speed which is the lowest speed that they can be operated under full power for a long time. Think SD-40 and variants and a MCS of about 12 MPH. AC units do not have the MCS limit, they can be operated all day at 2 MPH without damage. This is big reason carriers are careful about mixing AC and DC units in the same consist since crawling up the hill at 5 MPH will damage DC traction motors. Of course if a train has enough HP, it can get up the hill at 12 MPH and all is well, but that negates the idea of absolute minimum HP on drag freight. BNSF operates 110 car export grain trains over Marias Pass MT 1% westward ruling grade. How much power do they need? The grade is 20#/ton resistance and everything else is say 4 #/ton. Train weighs 110 X 143 or 15,730 tons. AT 24 #/ton need Tractive Effort of AT LEAST 377,000 pounds. With SD-40 at 90,000# need 4.18, actually 5 units. With modern AC at say 142,000# TE I need 2.65 so I assign three and take the excess in speed greater than what the units were rated at.  Couplers are all grade E which are rated for 360,000# IIRC. The power can put out more than that but train resistance should not be more than 377,000#. All involved hope there are no bad knuckles in the head end or the crew will find it and tie up the line for an hour or so, in good weather, to fix it. In practice the units will be rated at 5900 tons. Roundhouse foremen, dispatchers and crew will find they have capacity of 17,700 tons and a train of 15,730 tons so they are good to go. Of course since they do this day in and day out no need to really think about it until or unless the railroad is power short or a unit dies or some other disturbance occurs. Actual units are usually selected by Roundhouse Foreman based on expected tonnage from yardmaster, dispatcher, or centralized power desk. Practice varies by railroad, by locality, and by time in history. What the computer spits out is load, empties, tons, and feet based on what the clerk shows on the train which is based on what yardmaster and switch foremen told the clerk. Train will be scanned out of the yard which should correct consist errors. Mac PS at 8:32 PM IIRC BNSF is actually operating DPU mode, 2 on the point and one on rear, to limit draft forces in head part of the train. Mac [/quote] Thanks for your thorough and readable explanation.
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