Why are train tracks flat?

I know it sounds dumb, but they have such difficulty going up the slightest of grades. If they had teeth (like gears) than they could kick some serious ass.

I know its probably not cost effective, but hey… what do you think?

Funiculars have two railway carriages, usually on two parallel tracks on a steep incline, and usually designed so that the floors of the carriages are level. They are attached to either end of the cable and powered by a winch, so that when one carriage goes up the incline, the other goes down.


You mean like cog railways?

Pikes Peak in Colorado has a cog railway to take tourists up to the top. I road it once a long time ago. It was a fun ride, but they make you go back much too quickly.

Rail vehicles can go up/down slopes no problem, maybe with the exception of very long/heavy freight trains.

Remember that the wheels are locked together, they spin at the same speed. So it’s going to be a problem to have teeth in curves where the wheels routinely slip on one side. And it’s going to be hard to design a system with some parts teeth and some parts no teeth.

The whole point of a train is to carry very heavy weights at a minimal cost. Steel wheels on a flat track gives you a fairly low amount of rolling friction. Lower rolling friction means lower fuel costs. Flat tracks are also fairly easy to produce and maintain. Any sort of ridges that you might put on the tracks and wheels to increase the grip going up hill will increase the rolling friction, will cost more to produce, and will wear down quickly and will need replacing much more often.

Most of the country is flat enough that you can find routes to place your tracks on that don’t have huge inclines, so the lack of hill-climbing ability is no biggie for the big freight trains.

In the rare cases when hill-climbing is needed, there are ways of doing that, as described by the previous posters. Most of the time it’s not needed.

Cog railways don’t typically use two toothed rails - they use two smooth rails and a single additional toothed central rail (aka a rack), as in this photo.

And train wheel do not routinely slip in curves - they have a tapered rim that yields a larger effective diameter for the wheel on the outside of the curve, and thus no need for slipping.

If this were wanted, it would be reasonably easy: just put the toothed third rail where needed (steep sections) and omit it elsewhere.

The wheels are not locked together. They turn independently, just like an automobile. Why waste energy pulling a slow wheel through a curve. There is no brake on an axle. Each wheel has its own brake. Power wheels have a differential.

Even for passenger trains, a slope is no easy matter. The California Zephyr (San Francisco to Chicago) needs two engines on that portion west of Denver and the grade never exceeds 2.5%. The 40 km train ride from Visp to Zermatt, Switzerland, rises 2000m. About 10% of the track is a cog railway and the train labors quite a bit during steep climbs.

This is definitely not the normal practice. Pretty much all normal railroad wheelsets consist of two wheels rigidly fixed to an axle.

From this Wiki article:

The amount of force that a set of wheels can apply to a surface before slipping is determined by the coefficient of static friction between the two materials. (Basically, you multiply the vehicle’s weight by the number given.) For steel on steel, this number is in the 0.5–0.8 range; for rubber on asphalt, it’s about 0.72. So two numbers are not wildly different with each other; the effect of frictional force between a car and the road is not that different from the effect of frictional force between a train and its tracks. The reason that trains don’t accelerate very quickly (and can’t climb very steep grades) is not that they will slip otherwise; it’s that they have an awful lot of mass.

It is true that a steel ball bearing will roll for quite a while on a hard surface (longer than, say, a rubber ball with the same initial velocity.) However, I think this is largely due to the fact that the steel ball bearing is more rigid and isn’t constantly sagging under its own weight as it rolls, so it loses less energy to deformation. It’s not a “frictional” force per se (if we define “friction” as a force between two surfaces), even if it does act to slow down a rolling object.

And, depending on the type of train, only a few axles are powered. One locomotive with 4 driven axles can pull more than a dozen railcars, each with 4 unpowered axles.

High-speed passenger trains, light rail and subways typically have more powered axles - sometimes all powered axles. This allows better acceleration and steeper grades. Here’s a list of the steepest conventional railways - the steepest being a 14.5% grade (!).

I’d also suggest that the lack of steep grades is one reason railways are so efficient. This, combined with the very low rolling friction, allows a large amount of cargo to be moved with very little energy. If you equipped every freight train with enough power to climb a 15% grade, it would be far more expensive to operate.

But that’s what rolling friction is. Rolling friction is caused by deformation of the surface and/or the rolling object. Which is why increasing tire pressure reduces rolling friction - it reduces the deformation of the tire.

Correct. The taper is also why the top of the rail is curved; it needs to have a single point of contact with the taper of the wheel. The wheelset will tilt into the curve at just the right angle such that the difference in radius from the taper exactly accounts for what the curve demands.

On a related note, the flange on the inside of the wheel is not used in typical operation. The taper is what keeps the train centered. Even when going in a straight line, natural variations in the track would cause a train to drift to one side or the other if there weren’t a taper. Instead, real trains naturally “steer” back to the center if they are perturbed slightly.

Although the fact that relatively few drive wheels have to pull that enormously massive train also makes it much different than a road vehicle. As I understand it, in some circumstances locomotives will dump sand on the tracks in front of the wheels for extra traction.

This was an interesting discovery I made recently. Train wheels are slightly tapered from the flange side outwards, which tends to make the wheel push inwards and away from the flange. The whole effect is somewhat like a car riding along a V-shaped road. The flange is only there as a safeguard. If it were constantly hitting the track, the train’s rolling friction would be pretty high.

From Wikipedia’s article “Sandbox (Locomotive)”:

More primitive systems involve a couple of fellas sitting at the front of the locomotive dumping sand on the tracks out of sandbags.

So the pennies don’t fall off.


Yep. You would hear squealing all the time if there were slip; instead you only hear it on very sharp turns.

I learned this curious fact from the inimitable Richard Feynman.

That’s a great video. I’m going to watch more of those. Here’s one I just found for the more visual learners among us: http://youtu.be/agd8B-31bjE

There’s a cog rail up a hill in Budapest and also one to the top of Montserrat outside Barcelona. Cog railways are pretty common on steep routes.

Yes, there is less energy absorbed if not squishing a soft deformable rolling object. Tires are useful not because the have low friction but because they do deform - as anyone who has ridden a hard-wheeled conveyance can tell you. Even on smooth asphalt a shopping cart can be a rattletrap.

As for the OP’s question- simple answer - it’s not necessary. Wheel slip is not a problem, and typically the advantage of trains is very low rolling resistance’ which means it’s cheapest to run with very little power and make the track level… Sort of the opposite of an automobile, where in typical use the vehicle uses much less than its total engine power. As others mention, just add engines for the steep sections.

What do you mean ? They have no difficulty going up one in 10 million billion trillion grades

Trains were designed a LONG time ago. Like many things, if they were to be designed today, I’m sure things would be different and better. But we are stuck with the way things are.