There's basically two ways train wheels can operate, the flanges could be either "inner" or "outer".

Inner- and outer-style wheels

Switches can be made in equivalent ways for inner and outer flanges.

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We could expect that, just like some countries, as well as different railway companies within countries, picked up different rail gauges, they could have also varied between inner- and outer-flanges railways.

We could imagine that, just like today some countries have left-hand and right-hand traffic, there would be inner and outer style rail wheels. Except this is not the case, the "inner" style is almost universal. Almost, because the "outer" style indeed did exist in railways' debuts.

Richard Trevithick railway (England)

Is there a technical reason that makes the "inner" style preferable ?

  • 1
    $\begingroup$ Would the self-centering effect of canting the rail be affected by this? Maybe the flange location wouldn't make a difference, but I can't picture it either way in my head. $\endgroup$
    – hazzey
    Sep 11, 2019 at 13:48
  • $\begingroup$ The loading effects of the outer flange on the bearings compared to the inner flange... $\endgroup$
    – Solar Mike
    Sep 11, 2019 at 14:24
  • $\begingroup$ Could it for a "push" stress at wheel to axe joint during turns can be more easily withstand than a "pull" one? $\endgroup$
    – carloc
    Sep 11, 2019 at 18:10
  • $\begingroup$ I’m guessing it is to minimize the issue where on a turn, the outside wheel needs to travel farther than the inside wheel. Since the wheels are fixed to the same axle this causes the high pitched squealing often heard. $\endgroup$
    – Eric S
    Sep 11, 2019 at 19:49
  • $\begingroup$ Note you have reversed the cone shape of the wheels. That makes them highly unstable - they want to turn in the opposite direction to the track due to inertial force. The flanges aren't supposed to touch the rails in a curve. The curve is limited to a radius that the cones will roll around. The flange goes on the inside so the wheel can be parted from the mold and the bogie will follow the corner. $\endgroup$
    – Phil Sweet
    Sep 11, 2019 at 19:49

2 Answers 2


If you leave off the flanges and just use cones that are solidly connected to an axle, then cones that taper outward (get smaller as they go out) tend to be self-centering but undamped (or underdamped, I'm not sure which).

This is because if the axle is offset, the wheel that's to the outside has a larger diameter, and tends to drive the axle to turn toward the center. The "outer flange" design you should would do the opposite.

So the "inner flange" picture that you present is mostly not relying on the flanges to keep the axle centered -- it's the taper of the wheels that does that. The flange is there to damp oscillations, and for when the system is overwhelmed by circumstances.

(There's a nice video of this on YouTube, but I'm too lazy right now to find it -- try searching on "train tire design", or maybe those words with the word "stability" tossed in).


On a fast curve the outer flange ( on the inside of the curve) will lift off the rail and the train will leave the tracks. While an inner flange is pushed down onto the rail giving more stability.

  • $\begingroup$ This seems a bit cryptic, though it is correct. You mean that if the wheel at the inside of the curve lifts (whether it has an inner flange or an outer flange), the wheel at the outside of the curve will run off the track with an outer flange, but not with an inner flange. $\endgroup$
    – alephzero
    Sep 11, 2019 at 15:53
  • $\begingroup$ I don't see how it makes any difference. In a fast/tight curve in all cases the flange inside the curve will start touching the rail, and if the train goes too fast there's a danger of detailment. That's why trains have to run at appropriate speeds. In an "inner-flange" style like usual this happen on the outer rail, and in an "outer-flange" style this would happen on the inner rail. I don't see how this changes anything. The only difference is that the inner rail is typically a bit lower in curves. Is it what makes the difference ? $\endgroup$
    – Bregalad
    Sep 11, 2019 at 17:45
  • $\begingroup$ The center of gravity is high above the rails, so the centrifugal force pushes/rotates the engine and cars to the outside of the curve. This lifts the inside wheel flanges above the rails - they are the only ones holding the train on the tracks. If everyone drove at" appropriate " speeds , there would be almost no vehicle accidents of any kind. It takes a lot of words to state the obvious. $\endgroup$ Sep 11, 2019 at 19:40
  • 2
    $\begingroup$ The flanges are a kind of last resort. The conical shape of the wheels keeps the axles centred and compensates for the lack of differential on the axle which has both wheels fixed on the axle. (Without the conical profile the axles don't self-centre and the two wheels would be running the same circumference but on different path lengths while going through a curve.) $\endgroup$
    – Transistor
    Sep 12, 2019 at 17:04

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