Railroad Physics

[access bob]

Unlike toy trains, real couplers have no sideways forces holding cars together. i researched all the above after a ride where some kid was seriously worried because his trainset couldn't run in reverse at all.

I'm trying to get my head around the relationship between model trains and real trains, because I always have the same problem (cars just get pushed off the tracks) and I always thought those same forces must exist on real trains.

Why wouldn't they? If you have a curve with two 85' cars on it, they're going to be at an angle to each other, and the locomotive is going to be at a further angle. Isn't the pushing force of the locomotive on the first car going at least partially in a direction that's off the track? (In other words, the force is being directed in a straight line from the locomotive, but the track is curved.) And with the weight of additional cars down the line, wouldn't that force going in the "wrong" direction at some point be greater than the force available to overcome the inertia in the direction of the track (causing a derailment)? I mean, say you're pushing 50 loaded oil tanker cars, and your train is stopped on a sharp curve. That would seem like it would take a large amount of force to overcome the train's inertia in the direction of the track, but to overcome *one* car's inertia (and the force holding it on the track) in the straight-line direction going *off* the track would seem like it would require considerably less force.

(I would think the same would be true pulling, but somehow pulling seems easier on model trains, at least.)

Or am I just way off base?

your right and wrong at the same time. Real railroad curves are very very gradual compared to model railroads, some of the very sharpest speed restricted curves on a real railroad would scale out to a radius of many many feet on even an HO model railroad.

for example on the NEC at milepost 355 (from Boston) the 125mph pushed curve is 0degree 10 minute. this would be about 57,000 ft Radius. can you imagine using that in HO guage. even if scaled to 1/87th it would still be a 655ft radius curve..!

and that is the reason that trains can be pushed in real life but not in models. also consider at 1/87 scale a passenger car would weigh over 1 ton. now the weight does not scale proportionally but even 1/87X1/87X1/87 or almost 1/2 a lb, not many half pound HO cars.

physics doesn't scale down very well...

Bob

 

[TimePeace]

Cool Discussion -

A lot of this is beyond me because I don't know the terminology. But I am reminded of a cool article I read last year in the New Yorker, by John McPhee (I always read anything by him because I love his writing style), about huge long coal trains out West. What was amazing to me was that it described a control system whereby on long heavy trains going up over a big hill, the engines and front cars are braking DOWNHILL at the same time as the rear cars are being powered UP the hill.

Anyone care to comment on this with more detail?

Best to all,

David

 

[PetalumaLoco]

Cool Discussion -
A lot of this is beyond me because I don't know the terminology. But I am reminded of a cool article I read last year in the New Yorker, by John McPhee (I always read anything by him because I love his writing style), about huge long coal trains out West. What was amazing to me was that it described a control system whereby on long heavy trains going up over a big hill, the engines and front cars are braking DOWNHILL at the same time as the rear cars are being powered UP the hill.

Anyone care to comment on this with more detail?

Best to all,

David

No comments on the physics of it all, but I also love reading McPhee. I think I ran out of his books, shame.

 

[the_traveler]

about huge long coal trains out West. What was amazing to me was that it described a control system whereby on long heavy trains going up over a big hill, the engines and front cars are braking DOWNHILL at the same time as the rear cars are being powered UP the hill.
Anyone care to comment on this with more detail?

This was also shown in the episode of "Extreme Trains" on the History Channel. That episode is to repeated Sunday (11/16) at 10 PM Eastern and Monday (11/17) at 2 AM Eastern.

 

[had8ley]

about huge long coal trains out West. What was amazing to me was that it described a control system whereby on long heavy trains going up over a big hill, the engines and front cars are braking DOWNHILL at the same time as the rear cars are being powered UP the hill.
Anyone care to comment on this with more detail?

This was also shown in the episode of "Extreme Trains" on the History Channel. That episode is to repeated Sunday (11/16) at 10 PM Eastern and Monday (11/17) at 2 AM Eastern.

Yes it is called "distributive power" and is controlled from the engineer in the front of the train via radio controlled signals. It order to keep such heavy and long trains together in one piece you have to keep the slack bunched as much as possible while still pulling loads over a hill. Break-in-two's occur less frequently than when all the power is not on the head on. You may have 7 or 8 engines pulling but the exertion on that first draw bar is astronomical. It's a lot better to "distribute" both power and dynamic braking forces throughout the train and avoid heavy exertive forces.

 

[wayman]

Yes it is called "distributive power" and is controlled from the engineer in the front of the train via radio controlled signals. It order to keep such heavy and long trains together in one piece you have to keep the slack bunched as much as possible while still pulling loads over a hill. Break-in-two's occur less frequently than when all the power is not on the head on. You may have 7 or 8 engines pulling but the exertion on that first draw bar is astronomical. It's a lot better to "distribute" both power and dynamic braking forces throughout the train and avoid heavy exertive forces.

Every CP train I saw in British Columbia this summer had engines in multiple places. (And there were a lot of them--generally spaced about ten minutes apart in both directions through Fraser Canyon. So if you're driving uphill at 50mph, you'd see a downhill train every few minutes, and if you pulled over to a scenic overview and stood around for twenty minutes, you might see two in each direction. Amazing railroading spot.)

Sometimes it was (1-2)engines, ~100 cars, (1-2)engines; sometimes it was (2-4)engines, ~100 cars, (2-4)engines, ~100 cars. (I don't think I ever saw a train with engines fore, middle, and aft.) I don't recall a tendancy towards one configuration being uphill and the other downhill, but they were always one or the other. (I'm pretty sure I did see some of each in both directions.) The only exception was the Rocky Mountaineer passenger train, with engines at the front only (downhill, south of Kamloops where both sections are joined).

I never thought to wonder whether the engines in different places might be providing different power or braking functions at the same time! This is fascinating stuff.

At Tehachapi maybe 15 years ago, I saw an SP oil train with 7 engines, ~120 cars, 6 engines, ~120 cars going uphill. That was one heckuva train, by far the longest I have ever seen.

And in Worms, Germany, some years later I had a fascinating conversation with a DB engineer whose lifelong dream was to be the engineer of a train like that through Tehachapi.

 

[Guest_Yerry_*]

My comment about model trains (only) having a sidewards force holding them together, I should have been more descriptive. YES, subsequent poster's remarks about long cars' couplers sticking out are true-- it's the stringlining and tipover examples I mentioned first. It's exacerbated by the arrangement of 86' cars-- photocopy a model of a 40ft car to the same size as an 86ft car, and the trucks are closer to the ends than those on a scale model of an actual 86ft car. SO, 86ft cars have MORE than TWICE the coupler overhang than a 40ft car.

The sideways force I was referring to comes from the spring in the model car's coupler, and is present even with straight cars. Straight-on REAL cars don't have either the spring OR this force. I've seen quite a few less-than-perfect HO scale layouts where pushing more than one car at a time through a turnout was neigh impossible-- sideways force combined with rough track.

More recent post; only the Auto Train has slack run-in and run-out. the older P40's were preferred for this train by engineers, as the mandatory blended braking system of the P42 made stretch braking much more difficult to do.

Before distributive braking, ATSF and UP had many places where a long train could be going UPHILL and DOWNHILL at the same time with the engineer having to make do with ONE braking choice-- and to complicate things, having to account to the time delay in the air brakes from the front of the train to the rear. AND, if that wasn't enough, releasing brakes can have an additional delay-- not in brake speed but in the locos creating enough high-pressure air to do the job. BN has used "braking repeaters" in the Cascades (dunno if they still do) ; older locos that are there solely to generate air. Railroad air compressors aren't small devices: they're V-6, 8 or 10 cylinder jobs that run in double and triple stages, with substantial radiators to take away the substantial heat generated.

 

[Amtrak Watcher]

Go to the link for some basic information on this rather complex topic: http://en.wikipedia.org/wiki/Push-pull_train