Okay Dopers… I know there are a lot of train fans (and at least a few physics majors) out there. I need help with a pretty basic question I’ve always wondered about. Where I live there is a commuter train that travels north and south on two parallel tracks. There are usually 4-5 passenger cars and 1-2 locomotives. There apparently is no turn around at either end so in one direction the locomotive(s) are “pushing” the other cars and in the other direction the locomotive(s)are pulling them. My question is simply which is easier… and by easier I mean which way is more efficient… fuel-wise. My experience is that it’s a lot easier to push a heavy object than pull it but anecdotes are always right.
It takes the same amount of energy either way.
My guess as to why its easier for you to push then to pull is that is because that the way the human body is built…but the same amount of “work” is getting done.
Sound OK? Or am I way out on this one?
My smart-ass answer is…it doesn’t matter.
Assuming there are potential savings re: pushing vs. pulling, these savings are offset by the greater expenses of:
a) turning the train (or even swapping ends with the locomotive) at the termini;
b) having a locomotive at both ends, so that the train always pushed.
Overall, making aerodynamic improvements would be much more beneficial in reducing energy use. Most US commuter stock are barn doors on wheels.
From the standpoint of dynamic behavior, pulling is better, with less potential for derailment.
High-speed trains such as the French TGV do have electric locomotives at each end, but this is due to the need for high horsepower to maintain track speed, and to provide increased protection to the passengers in a collision.
I would think that pulling is easier, because that’s the way they do it when they have a choice. But compared to freight cars, passenger rail cars weigh next to nothing, so it’s probably not a big deal.
Pushing might use a little bit more energy, but not enough to be noticed. I imagine the couplings between cars are designed to be under tension and running them under compression might generate a tiny sideways force (If the coupling isn’t straight, pushing would tend to make it bend one way or the other. When pulling, the coupling would tend straighten out). A little more friction, a little more heat, a little wasted energy.
The main reason they prefer to pull is to have the engineer in the locomotive. Otherwise you need a control console in the passenger car at the front, not to mention the electrical wiring to control the locomotive from the other end of the train.
I don’t think there is any energy saving - the rolling resistance and wind resistance are the same regardless of which way the train is moving. Electric motors are equally efficient in both directions. Well, the wind resistance will be slightly different if only one end of the train is streamlined, but that’s pretty rare.
Ooh, good point, actually. But I’ll still stick with my original answer.
Think of it like a car with a trailer. You can drive forward, no problem. But have you ever tried backing up with a trailer? When the trailer gets a little bit out of line, and you keep backing up, the angle gets worse until it folds up like a jackknife. The flexibility in the train couplings would act like the trailer hitch on the car. The wheel flanges would keep the train from getting more than an inch or two out of alignment (actually, the wheels are tapered so they should self-center before the flanges make contact with the rails), but I think you’d lose a tiny bit of energy to the extra friction when that happened.
On the commuter trains in the Toronto area, the locomotives stay permanently at the east end of the train. There is in fact a control cab in the passenger car at the west end of the train.
The trains are pushed when they go west, and pulled when they go east. All commuter-train routes are essentially linear, and converge to pass through Union Station downtown, so nothing ever gets turned around. Sometimes on a long train you will see a locomotive at each end, but then again I’ve seen trains with two locomotives at the east end only.
There’s already a lot of electrical and pneumatic connections to heat, power, and brake the cars from the locomotive; presumably adding control circuits to the other end wasn’t that much more hassle.
Now freight and intercity passenger trains seem always to be pulled, although, come to think of it, I saw a VIA intercity train with a locomotive at each end today. I’ve also seen VIA trains with a locomotive at one end and a locomotive in the middle standing in Union Station. My guess is that this train was heading eastwards and would split somewhere along the way…
Metro-North trains in New York are all electric, so there are no locomotives per se. It seems to me that each car has its own engines and brakes, all of which are controlled by the engineer in the front car. The rear car is attached in the opposite direction so the engineer will be in the front when going the other direction.
the LIRR new bilevel trains have a locomotive on one end and a passanger car w/ a ‘control booth’ on the other.
I assumed the dissel engine generated electricity to drive the electric motor at whatever end of the train was the front.
The Tri-Rail commuter trains in Florida’s Dade, Broward, and Palm Beach counties have a locomotive on one end and a control cab at the other. The engineer moves from the locomotive end to the “control cab” end to change direction.
I do not know if the individual cars are powered.