DPU at Lucin... 20091011_7073
This distributed power unit (DPU) brings up the rear of a westbound manifest freight at Lucin, Utah. In the United States the use of distributed power is on the increase, especially with Union Pacific Railroad. One advantage of having a pushing locomotive on the rear end is the reduction in the tendency of long trains in curves to "stringline."
Stringlining is the tendency of a pulled flexible thing (string, rope, hose, train) to want to form a straight line ("straight as a string"). Obstructions can prevent stringlining. Dragging a hose around the corner of a house or past a tree can force the hose to assume a less-than-straight line: the hose does not stringline. But without the obstructions, a dragged hose will be fairly straight (it stringlines).
A train is subject to the same physics, but instead of going around a house or tree, it travels through curves, which act as obstructions. While negotiating a curve, a freight car wants to move toward the inside of the curve. In fact, it is being pulled into the curve. Rails and wheels are designed to keep the cars on the rails in spite of this force, but the right combination of forces can overcome the best-designed track, resulting in a train's stringlining (forming a straight line in a curve).
The distribution of weight in a train plays a role, as does acceleration from a stop. Suppose a long train is stopped on curving track. There are three or four locomotives up front and none farther back. The first half of the train is empty freight cars. The rear half is loaded cars. When the stopped train starts up, two major forces are at work: the pulling force of the locomotives and the resisting (pulling) force of the loaded freight cars. Just as if a ground stake pops out of the ground, allowing a dragged hose to straighten out, the relatively weak holding force of the unloaded cars is not sufficient to keep them on the rails, and the train stringlines (derails). Starting a train slowly can counteract this problem. Assembling the train in a way that avoids the grouping of empty cars ahead of a grouping of loaded cars is a better solution but not always possible.
Another solution is to reduce the amount of pulling force from the rear of the train. That's what a DPU does. Thus the cars in the middle of the train are not pulled from both ends and they stay on the rails. (This description is simplified and incomplete, but it gets the idea across.)
The comments prompted me to add that DPUs are also used to reduce the strain on the drawbars and couplers of the locomotives and the cars near the front of the train.
The September 2010 issue of "Trains Magazine" had an informative article on the use of distributed power. The more I learn about railroads the more I realize I have much to learn.
DPU at Lucin... 20091011_7073
This distributed power unit (DPU) brings up the rear of a westbound manifest freight at Lucin, Utah. In the United States the use of distributed power is on the increase, especially with Union Pacific Railroad. One advantage of having a pushing locomotive on the rear end is the reduction in the tendency of long trains in curves to "stringline."
Stringlining is the tendency of a pulled flexible thing (string, rope, hose, train) to want to form a straight line ("straight as a string"). Obstructions can prevent stringlining. Dragging a hose around the corner of a house or past a tree can force the hose to assume a less-than-straight line: the hose does not stringline. But without the obstructions, a dragged hose will be fairly straight (it stringlines).
A train is subject to the same physics, but instead of going around a house or tree, it travels through curves, which act as obstructions. While negotiating a curve, a freight car wants to move toward the inside of the curve. In fact, it is being pulled into the curve. Rails and wheels are designed to keep the cars on the rails in spite of this force, but the right combination of forces can overcome the best-designed track, resulting in a train's stringlining (forming a straight line in a curve).
The distribution of weight in a train plays a role, as does acceleration from a stop. Suppose a long train is stopped on curving track. There are three or four locomotives up front and none farther back. The first half of the train is empty freight cars. The rear half is loaded cars. When the stopped train starts up, two major forces are at work: the pulling force of the locomotives and the resisting (pulling) force of the loaded freight cars. Just as if a ground stake pops out of the ground, allowing a dragged hose to straighten out, the relatively weak holding force of the unloaded cars is not sufficient to keep them on the rails, and the train stringlines (derails). Starting a train slowly can counteract this problem. Assembling the train in a way that avoids the grouping of empty cars ahead of a grouping of loaded cars is a better solution but not always possible.
Another solution is to reduce the amount of pulling force from the rear of the train. That's what a DPU does. Thus the cars in the middle of the train are not pulled from both ends and they stay on the rails. (This description is simplified and incomplete, but it gets the idea across.)
The comments prompted me to add that DPUs are also used to reduce the strain on the drawbars and couplers of the locomotives and the cars near the front of the train.
The September 2010 issue of "Trains Magazine" had an informative article on the use of distributed power. The more I learn about railroads the more I realize I have much to learn.