I have always wondered, why do railroads, when there is more then one locomotive being used, place the second locomotive so that it appears it is running backwards?
They don’t. It happens that way.
And when they do, its so they don’t have to turn the engines for the return run and for ease of switching.
Diesel electric locomotives perform equaly well whatever direction they are faceing (and the front is not always the end the cab is on). The important thing is it is very desireable for crew comfort / visibility to have the lead unit faceing the right direction. All the others engines can face whatever direction they happen to be faceing when the consist is built. Now there are many situations where there are no turning facilities like wyes. So then when a train runs to the end of the line the consist will change directions for it’s return trip and run around the train. Then the unit that was at the rear now is the lead unit. So it’s desireable to have the two end units in a consist face the right direction. Other then that the middle units can face any direction.
On mainline trains like an intermodial from Chicago to LA only the lead unit needs to face forward. It doesn’t matter which direction the rest of the units face. The exception would be if the lead unit fails and has to be set out. Then it would be desireable to have the 2nd unit face forward as it will assume the roll of lead unit. But if there is no other unit faceing forward then the crew may have to run long hood forward. That is very undesireable as the engineer will be on the wrong side and faceing backwards.
Note: Some railroads (Southern,N&W) prefered to buy there locomotives setup to run long hood forward for extra protection for the crew in collisions. Some of these were setup with two controll stands so they could be run in either direction and still have the crew members in there proper respective sides of the loco.
Based on that, are there the same number of “gears” for forward and reverse or is it just a throttle forward and a throttle reverse? Also, when more then one engine is being used how are the speeds syncronized between the engines so a locomotive that may be second or third, doesn’t try to move faster and push a lead locmotive off the tracks?
Thanks!!!
I’ll leave the details to the pros but I’ll try to provide some basics. The throttle on a locomotive is electrically linked to the engine-alternator set. The mechanical output of the engine and the electrical output of the main alternator/generator are carefully matched and linked by the control system. Direction of travel is governed by the reverse lever. The throttle controls the speed of the engine and generator output. In multiple unit control, the locomotives are electrically linked by jumper cables so that the throttle in the cab of the lead locomotive controls all locomotives.
Some locomotive models will load a little faster than others so the individual units in a multiple unit consist are not perfectly synchronized.
The only gears involved are the gears on the traction motors that mesh with gears on each axle. They are permanantly enguaged (unless disasembled for maint) and are typicaly in the ratio range of 62:15. Changeing direction is simply changeing the polairity of electricity to the traction motors. There is no mechanical gear change.
When multiple locos are used each one contributes it’s traction to the train in accordance to it’s ability. It’s like if a father and son are both tugging on a rope. The son might only contribute a portion to the bottom line but but he will add that much more pull to the fathers pull. So the different locomotives don’t “fight eachother” but compliment eachother.
Not all locos are geared the same. Passenger locos typicaly have higher gearing.
It’s a balance between the top speed needed and haveing enough power at lower speeds. You can only spin a traction motor so fast before it fails in a spectacular cloud of smoke and sparks with a big boom. On the other hand you have to maintain a certain traction motor speed under full load to keep them cool enough. That’s where the term short time comes into play. You can dip below the minimum traction motor speed but only for a short time. On the ampmeter there is usualy a series of short time marks that start at around 1000 amps and go from “continueous” “1/2 hour” “1/4 hour” “ten minuets” and “five minuets”. A typical DC freight unit has a min continueous speed of about 12 mph and has a upper limit of 70mph. A passenger unit can have a minimum continueous speed of 18mph and a upper limit of 100.
Now you can run a freight unit with a passenger unit and they will both contribute to the bottom line, But, the freight unit will pull more then the passenger unit because it is geared lower (thus haveing more torque multiplication). The problem is you can’t drop below the min speed of the passenger unit (18+/-) or you will overheat the traction motors, and you can’t go faster then the max of the freight unit (7
regross – it can be hard to visualise how the various units in a multiple unit setup cooperate, particularly if you come at it from a model railroad point of view, where the drive train is anything but free-running. The main point is that in the real world, application of power to a traction motor produces a torque, which tries to turn the wheel. Just how much torque is developed is set by the traction motor gearing and the setting of the engine controls (‘notch’). But each axle (traction motor) of each truck of each engine will contribute some torque and thus drawbar pull to the whole thing. I like the illustration of the tug-of-war!