Determine the placement of locomotives in a freight train

[Railman37]

Hello all,

This is my first post on this forum. I've read alot of posts and I like the friendly community here on this forum.
Also I have to mention that I'm not from America, I'm from Europe (The Netherlands). English is not my native language, but I think I can manage to understand/explain quite a bit.
I have a fascination for American trains, especially goods/oil/coal/mixed freight trains.
Also I am not entirely up to date with the jargon, but I'm learning. Please forgive me or correct me when needed.

I understand that the number of locomotives used to pull a train can be determined by the total weight of the cars and how many grades and the percentages of the grades, type of weather, economics, costs, etcetera. Maybe a bit of a simplified view on my part, but in general you could look at it in this way.

My question is how does one determine where in the train the locomotives are used?
example:
Lets say we have a loaded 100+ cars coal train. I've seen trains that only use locomotives in front to pull the train, but also other trains with locomotives in the middle (dpu) and/or at the end of the train (helper).
When is it decided that locomotives are added to be used as dpu units or as helpers at the end of the train ?
Does it depend on how much force there is on the couplers or how steep grades are or something else ?

It would be great if you could share your thoughts about when to use dpu's or helpers or only locomotives in the front.
I hope I've put this question in the right section of the forum. Again, my knowledge about American freight trains is somewhat limited.
Thanks for reading and hopefully reacting

[Ericmopar]

It's based on drawbar pull and grades mostly. A coupler's drawbar can only take so much stress, so they spread the power out accordingly.

[buzz456]

In the real world sometimes in the cold weather they put a loco mid train just to keep the air up.

[BillS]

Your English is much better than my Dutch, and way better than many people that use English as their primary language. Welcome aboard.

[Railman37]

Thank you Ericmopar / buzz456 and BillS for your comments, much appreciated!

So in general you could say that the position of locomotives within a train is based on drawbar pull, grades and/or weather conditions.

About freight trains in The Netherlands (Holland):
We do have freight trains here of course, but not as impressive as in America.
Usually freight trains overhere have a total of 20 to 40 cars and are always pulled and locs are never used as dpu or pushing at the end (with some exceptions and passenger trains do use dpu's or push-pull combinations).
Also most (if not all) of the locomotives used overhere have a cab at both ends and are electric, with the exception of some yard switchers or diesel locs.
Tracks here in Holland are double tracks (most of them) and electrified (overhead catenary wires with a wopping 1500 volts! ) (this is not a typo, its fifteen hundred).
Anyway, I digress from the subject.

Thank you for reading and again thanks for the welcome! I'm sure to check Railworks America forum frequently!

[GEtrack]

The knuckle in the coupler is designed to be the weakest link. I forget the exact values but there was one for regular freight trains and a stronger one for "unit trains" (coal for example). The railroads always set the limit based on the number of "powered axles" at the head end, so if all the power had six axles you would divide the number allowed by six and round down to get the number of locomotives. Or if you were just short rounding up, you could turn off power to one of the traction motors in the cab. If you needed more power to make the ruling grade they would be added as pushers (at the rear) or "swing helpers" somewhere in the middle.

[BNSFdude]

The knuckle in the coupler is designed to be the weakest link. I forget the exact values but there was one for regular freight trains and a stronger one for "unit trains" (coal for example). The railroads always set the limit based on the number of "powered axles" at the head end, so if all the power had six axles you would divide the number allowed by six and round down to get the number of locomotives. Or if you were just short rounding up, you could turn off power to one of the traction motors in the cab. If you needed more power to make the ruling grade they would be added as pushers (at the rear) or "swing helpers" somewhere in the middle.

There's type E, standard on most equipment. Then there's type Fs which are used on some grain hoppers, and most if not all coal hoppers and ore hoppers. Type E's are rated at a maximum of 360,000 lbs continuous, 500,000 shock (slack) load.
Type F's are 410,000 continuous, 515,000 shock.

The rated powered axles is actually much more complicated than you stated. It's based on actual tractive effort. Here's ours on BNSF:

[Railman37]

Thank you GEtrack and BNSFdude for the indepth comments, this is very usefull for me to better understand how US freight trains operate or are assembled. I appreciate the chart also ! thank you very much