Why do freight trains sometimes have locomotives at the back?

Question

Why do freight trains sometimes have locomotives at the back? Over the past couple of days I saw a bunch of freight trains that had a few locomotives in the front and also a couple in the back. What's the point?

Answer 1

as stated above they reduce strain on the couplers

this is also beneficial to the rail structure, you can imagine the tremendous stress on the inside rail of a curve with 10,000 or more horespower all pulling as hard as they can to get up a grade.

Putting some of tha power on the rear not only reduces wear and tear on the rail, (which is VERY expensive to replace) it reduces flange friction between the wheels and the rail which makes the train easier rolling.

I remember an old story, about a train exactly the same size and number of cars leaving a mine each day, they put two locomotives on the point and went up out of the mine at 12 miles an hour, one day they tried putting the same two locomotive one on the point and the other one on the rear and went 2 MPH faster because of reduced flange friction. In mountain grade railroading that is pretty significant, especially when you consider it was the same tonnage and same horsepower.

Sometimes manned helpers will shove on the rear of a train instead of "cutting in" to the middle to help a train over a grade, this is much faster than breaking the train apart twice, once to cut in and then again to cut the helpers back out.

Answer 2

In the UK these locomotives are called, 'bankers' and were once a common sight on lines where the grade is uphill 1:50 or steeper.

Answer 3

These are called "pushers".  They are usually added to the train to assist in areas with steep grades.

Answer 4

Pushers are controversial when it comes to passenger trains. In southern California there have been some horrific accidents and people blame pushers for increasing the severity of the damage done to the passenger cars.

Answer 5

They are called DPU or distributed power units.Trains can be pushed from the rear and use less power to move the train than if all the power is at the front.It helps distribute forces generated to the track and the couplers also.It's much easier to push something than it is too pull it.We run 20,000 ton coal trains on the line i work with just 5 units,3 on the front and 2 on the rear.If you tried to pull that much weight with them all on the front chances are good it would break in two from the draft forces.Draft is the force put on the couplers from the locomotives pulling the cars.The rear units are run by the engineer in  the lead engine by remote control.

Answer 6

To reduce the force on the couplers. Trains in the US are often 10.000tons or even heavier; if you put all the engines to the front the first couppler (between engines and first car) would have to withstand the total force to pull the whole train. If you put half of the locos to the rear to push the first and last coupler on the cars only have to absorb half the force.

Sometimes there also are locos in the middle of the train, to distribute the drag even more.