# Differentials on train axles

Axles on railway cars are solid, making the two wheels rotate together even when they go around curves. So when they do go around curves, one of the wheels is dragged, thus making a loud screech. Also they lose a fair amount of energy in the process.

Is there any reason they can’t put differentials on rail car axles? I’m thinking about light rail and street cars[sup]1[/sup] rather than box cars and such. Would they save enough energy to make up for having to haul around the extra weight of the differential?

[sup]1[/sup]It’s possible that railed street cars already have differentials, but I know the light rail cars around here do not.

Traincar wheels are tapered. They have a larger circumference on the inside tapering to a smaller circumference on the outside.

When a train goes around a curve, centrifugal force pushes the car to the outside of the curve, causing it to ride on the larger circumference portion of the wheel.

Meanwhile, the wheels on the inside of the curve are riding on the smaller circumference part. Even though the axle is turning at the same speed, the wheels are, in effect, different sizes, and this gives a differential effect.

This is correct. However, the system is very basic, and speeds are not matched perfectly, so you still get squeal on tight curves. On the other hand, much of that noise is “flange squeal”, which is different - even if the face of the wheel has perfect adhesion on the rail, and is silent, a tight curve can see the front part of the flange scraping vertically against the inside of the rail. If you look, you will often see huge grease guns on the track just before a tight curve, to grease the inside of the rail (not the running surface, of course) to help eliminate this problem.
Modern streetcars/trams don’t have differentials as such. They go one step further - they don’t even have axles. The wheels can be mounted completely separately within the frame, and individually powered.

Since most railcar wheels are not “driven”, can I assume what you’re really asking about is separate bearings for each wheel?

I would assume the answer has to do with the normally large radii of curves.

And that’s why developments in this area have come from the streetcar/tram/light rail side of things*. The heavy rail industry just puts up with a bit of wheel slip. It’s easier, cheaper, and when all is said and done, it’s not a huge problem. It’s there, but it’s no biggie.
Edited to note: admittedly, a big part of the elimination of axles in streetcars is to comply with low-floor requirements for modern disabled-access legislation.

Train nut here. The preceeding posts are essentially correct. The tread of a railroad wheel is about five inches wide and has a three degree taper. Then there’s a fillet leading to a flange which is only about an inch high. A wheel pair and axle rolling down straight track has both wheels at the same diameter so things continue straight. If there is a side thrust, say to the left, the left wheel is now contacting the rail at a slightly larger diameter than the right which moves the pair back to the right and equilibrium again. If there’s a severe side thrust, the fillet comes into play, lifting the wheel on that side and providing a tremendous force back to the center.

Because of the fillet, the flange rarely comes into contact with the railhead. Its purpose is to guide the wheel pair where there is a discontinuity in the railhead, as in a switch. The guardrail catches the back of the flange of the wheel passing through it, ensuring the flange of the wheel going through the frog follows the proper path.

On curves, as Zambini57 and TheLoadedDog said, the sweet spot is found where the two wheels are rolling on diameters where neither is dragging. On sharp curves, the diameter differences are insufficient, and that’s where the flange greaser comes into play.

I think it’s not reasonable that centrifugal force would be the mechanism for moving the wheel to a larger diameter on the outside of a curve and vice versa. For one thing, this would only be right for one speed. For another, at least some curves are banked.

More reasonable would be the wheels own tendency to steer that way, precisely because the wheels turn together.

The same mechanism keeps a flat belt riding on slightly crowned pulleys.

You can see this at work if you put two cones together, base to base, and try to wrap tape around them. Even if you twist kinda hard, the tape keeps wanting to track back onto the joint between the cones, and it’s very difficult to tape the cone surfaces away from the joint.

Note, I’ve not heard this anywhere, it’s just what seems reasonable to me.