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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.

enter image description here

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 ?

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    $\begingroup$ 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 ♦
    Commented 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
    Commented 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
    Commented 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
    Commented 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
    Commented Sep 11, 2019 at 19:49

3 Answers 3

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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).

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  • $\begingroup$ Maybe it's this one ? youtube.com/watch?v=agd8B-31bjE $\endgroup$
    – Bregalad
    Commented Sep 12, 2019 at 6:46
  • $\begingroup$ That's not the one I've seen, but it certainly looks good. $\endgroup$
    – TimWescott
    Commented Sep 12, 2019 at 14:57
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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.

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  • $\begingroup$ 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
    Commented 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
    Commented 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. $\endgroup$
    – blacksmith37
    Commented Sep 11, 2019 at 19:40
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    $\begingroup$ 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
    Commented Sep 12, 2019 at 17:04
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You need the conical profile (typically about 1:20) both to self-centre the wheelset and to enable it to go round normal curves without slipping- the cause of the wheel squeal you get on tight curves (qv the Central Line on the London Underground). Centrifugal forces make the wheel set tend towards the outer side of the curve and the taper then makes the "outer" wheel a slightly larger diameter (on a curve the track is also usually "canted" to reduce the centrifugal foces for both stability and passenger comfort). The inner flange is an obvious extension of the taper and on curves acts as a "backstop" to stop derailments but it also guides the wheelset through turnouts (points) where an unflanged wheel wouldn't "know" which path to take through them. You could do this with an outer flange but the forces would then be putting the wheelset into tension whereas an inner flange puts it into compression which is normally safer. A tapered wheelset with an outer flange would also be far harder to cast and, where separate tyres are employed (as they generally were on steam locos and are on many coache) you wouldnt be able to fit them by the normal method of dropping a heated tyre onto the wheel core (from the outside the flange would get in the way and from the inside the axle would)

Any one of those reasons would militate against outer flanges, taken together it's a no-brainer. However, double flanged wheels were used on some industrial railways (notably in quarries and mines) where the track was so roughly laid that the gauge wasn't certain so the wheelset incorporated loose wheels that could slide to and from on the axle (to a limited degree)to accomodate a somewhat varying gauge. They are also used where the wheels are not on opposite ends of a single axle - as for example on a travelling portal crane or hoist running on rails.

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