Motive Power Needs

[charlielima]

Could someone in the know tell me if there is a "formula" that railroads use to determine the amount of motive power required for train consists involving weight, track grade and speed?

Also, what determines whether a loco will be placed mid-consist or at the end of a consist?

These are answers that will help add that added touch of realism to any trainz session.

Thanks. CharlieLima

 

[cascaderailroad]

I am sure there is a formula. But when NS tries a new innovation, to run a hotshot intermodal with 125 cars with only 2 locos on the head end, the train must get a running start up the 11 mile mountain grade, in order not to stall. Usually experience, guestimation, and on the job training, helps dispatchers maintain that enough power is available for steep gradients.

Usually a intermodal has 3 locos head end with no helpers, and merchandise freights and coal drags have upwards of 4 head end with 2 more shoving on the rear. Also the 2 rear end helpers assit in dynamic braking, helping slowing the rear end of the train down the other side of the mountain grade. The Pittsburgh line does not use mid train DPU.

Since they know that the Altoona to Johnstown gradient is @ 1.25% they usually add more power over the mountain, and sometimes remove un-needed power at the base of the grade, on the less steep trackage: Johnstown to Pittsburgh, and Altoona to Harrisburg, which generally has a gradient of much less than 1%.

Usually a 6 loco train is arrainged: <--//<--//<--//-->_______train________<--//--> so reverse moves can be easily facilitated. Usually a engine shop hostler arrainges locos in sets of 2, with locos facing, opposing one another: <--//--> And added power is hooked on when needed. Sometimes a lashup of particular units stays together for months at a time, until a breakdown occurs, or power is pulled off a consist, and is swapped.

 

[Bob5446]

Charlie,

I am not an expert in this field but from what I understand there are several parameters at work. The obvious ones are the weight of the train and the grade it has to travel over. However there are other key factors as well which include the length of the train, amount of braking per locomotive axle, and the priority of the train. I believe that for example some railroads will always insert a dpu into a consist after 70 cars, or 7000 feet which explains why you won't see a dpu cut in exactly half way in a consist but more likely closer to the back, where there may also be additional power. Another variable is train priority - a z train will need to be sufficiently powered to run over a route at as fast a speed as is safely acheivable in line with track-speed limits and hills in the way so that it arrives on schedule. A low priority manifest for example does not need that extra speed - as long as it can travel the route in a reasonable time and get over any hills in the way that is good enough. Locomotives are expensive to buy and expensive to run and crew. There are formulas used by the power divisions of railroads to work out what each train requires in terms of horsepower and braking abilty.I couldn't tell you exactly what they are because of all the variables outlined above but I think it is fairly well accepted that a low priority train will be running at perhaps 1 to 1.5 hp/ton while a high priority train could be 3-4 hp/ton (or more).

As stated I am no expert and this only relates to what I have picked up from other sources over the years. I am quite happy to be corrected on anything by those with more knowledge! Hope that helps.

Bob

 

[Lots_Trains]

Assigning power to a train used to be worked out as "horsepower per trailing ton", these days both UP and BNSF use TPA (tons per equivalent power axle). Each loco as an axle rating, eg: a 6 axle SD70M is rated at 10.4 axles, a ES44AC is rated at 12.1, an SD-90MAC is rated at 11.5. So for a train traveling over a particular route at whatever weight will require X amount of axles.

Apparently DPU mode dosn't work or is not reliable over 8,000 feet, most DPU units goes on the rear of the train, From what I have seen of DPU units in the middle of trains is they are mostly on large Grain trains or Potash trains.

Cheers

Lots

 

[charlielima]

Hey, this is great. Thanks one and all.

From what you're all telling me, a hobbist engineer, like myself, has to know the tonnage and length of his/her consist before just slapping 5 or 6 locos onto the front of a train. This really makes running trains on the computer more interesting and more "fun".

Lot, I live in Winnipeg, Manitoba, Canada and work next to Canadian National Fort Rouge Yards and seeing a mid-consist DPU is quite common, especially on the intermodal trains that will usually have 2 C44-9Ws at the head-end and another CW or SD70M about, like you said, 70 cars back followed by another 30 to 50 cars (that's one looong train).

I've only seen one example of a CP intermodal train with two ES44's in front, an AC4400CW mid train and an AC4400CW facing rearward at the rear. That train was about 130 cars.

Thanks, again.

 

[Bob5446]

Charlie,

By chance the latest edition of Trains magazine arrived through my letterbox this morning and it has a very good article on the use of DPUs which might be of interest to you if you can get hold of a copy.

Bob

 

[RooRocz]

Sometimes I think they just slap locomotives on trains willy-nilly... here flat Mid-Atlantic Washington DC, I've seen four big angry modern locomotives on the head end of the NS local to Alexandria which usually only has ten cars and a caboose.