How do trains turn corners?

Question

When a vehicle turns a corner, one of the wheels moves faster than the other, as it needs to travel a greater distance in the same time.

Cars achieve this with differential steering.

Trains have solid axles, and I'd assume the friction caused by one of the wheels moving slower would be enough to melt the track, so how do they do it?

Answer 1

If you look at railroad tracks everywhere, you will notice there is not one 90 degree turn. When they turn its a slow gradual curve that makes them change direction

Answer 2

trains don't take that sharp of turns, yes it would have a little friction but not enough to really change the temperature of the tracks to the melting point of metal.

Answer 3

The wheels are not 'flat' but have the shape of cones. So if the axle moves to the outside the outside wheel runs on a bigger radius than the inside one.

This also helps to center the wheelset on an straight line. See http://en.wikipedia.org/wiki/Hunting_osc... for details.

Answer 4

They are guided by the rails. The surface of the rail is curved and the profile of the wheel is slanted. When a train goes around a curve, which in general is a lot shallower than a road bend, the track is tilted into the curve, which in turn allows the wheels to find their own level on the surface of the rail. Compared with a road vehicle only a tiny proportion of the tread is in contact with the rail at any one time. This reduces the friction to a minimum and also enables what friction there is to do the job it's meant to do - provide the adhesion necessary for the train to get its traction.

However, going round sharp curves does set up stresses between train wheel and rail which can be heard as metallic grinding noises as the train moves. The sharper the curve, the slower the train must move, yet even at slow speed the noises are clearly audible.

Therefore the aim of railway builders is to make track curves as gentle as possible.

Answer 5

Gentle radius.

Low friction to begin with.

Wheels are tapered to dissipate the energy, actually allowing for reduced flange bind by allowing the taper of the wheel to turn "faster" on the outside of a curve (it doesn't, of course, but the wheel taper fools the wheels), serving to a small degree as a "differential" found in automobiles.

Most railroads employ "curve greasers" or "flange lubricators" that automatically applies grease to each wheel from an underground reservoir at each end of the particularly tight curves.

In addition, curves of any radius don't just "start," as with a snap-track model railroad track.  They have engineered into them a "spiral" on each end.  This spiral starts the truck turning before the wheels attack the outside rail in the curve, allowing for a much smoother process with lower force levels.

In addition, curvature can be "canted" similarly to race car tracks or high-speed roadways.

Where there is tight curvature in heavy grade territory, curves are often "compensated."  What this means is that the grade, or amount of incline, is reduced slightly through the length of the curve. This reduction keeps the forces lower and, more importantly, uniform through the given area.  Force level "spikes" can cause drailments.

The determining factor is L/V, or the ratio between the "latteral" force and the "verticle" force.  As long as the latteral force (centrifugal force, if you will)) does not exceed verticle force (gravity)  then the wheels will roll through the curve with no problem.  If, however, lateral force becomes .9 of verticle force or greater, then the wheel will climb the outside rail of the curve in "buff" (slack bunched, as when descending a grade) or be pulled off the inside of the curve in a phenomenon known as "string-lining," when in draft, when pulling tonnage.

There are no "simple" aspects of railroad operations.  Everything is on stereoids (except the crews, of course).

A very good question.  Thanks for asking.

Answer 6

You're right to say that the wheel on the outside of a curve has to cover a greater distance, but curves on railway tracks are usually of large radius, so the difference is small compared to the distance travelled. If you look closely at a railway wheel, you'll see that, as well as having a flange on the inside edge, the wheel is slightly tapered. There is a diagram at http://www.rowehankins.co.uk/wfl/ On a curve, there is enough sideways movement in the suspension to allow the wheelset to run slightly "off centre" so the wheel on the outside effectively has a slightly larger radius, which takes care of the differential.

Answer 7

it moves the same speed it lins to make it a round the bend.

Answer 8

there are curves in tracks  and the wheels do pivot on every car and engine so when its times to turn they can.  there is no steering wheel in a train either.  also that is why there are couplers also because if the train was one piece than they couldn't turn either.

Answer 9

The first thing a railroad trackman will tell you is "there are no corners on a railroad, there are only curves"

I cant tell you how many times I have heard this, railroad curves are generally not real sharp so the problem is not as much as you might think.

You are absolutely correct to notice the wheels are on solid axles and the outer has to travel a farther distance but the curves are gentle enough it is not really a big problem.

In addition, the wheels are mounted on a "truck" or subassembly that pivots independently from the rail car or locomotive so in effect it is always heading in the direction of travel on the rail, not the direction the actual car is facing, this further reduces the amount of friction between the wheels and the rail.

Another thing in railroad's favor is the fact that there is far less friction between polished steel parts than in rubber tires and asphalt highways.

this is the reason railroads can move so many more tons of freight for each gallon of diesel fuel than a truck.

Good question, very observant.

Answer 10

go to your local Hobby shop with trains. have them run for you or show you any engine that should be a good repetition of it that's why it's model. but look at the trucks and how te move

Answer 11

The radius of the corner is so great it would almost be a straight track.  Also if there was enough friction to melt the train's wheels it would be a very poorly made train!!  You think of the enormous weight the wheels have to support... there is friction between them and the track and they don't melt.

Answer 12

there are these things called flanges on the wheels that keep the train on the tracks.  when the locomotive and cars reach the corner u may here a screech sound.  the trucks which are the wheel sets can turn so its not fixated straight all of the time.

Answer 13

Trains don't technically turn. The track guides the train to where it moving to. There is some friction while this happens but there is actually little contact between the rail & the wheel. That type of steel the tracks are made of is very difficult to melt.