Why do trains only have a single gear?


Trains accelerate incredibly slowly and often have a single gear that is optimised for high speed. Similar to trying to pull away in your car in too high of a gear this makes trains really slow and takes along time for them to hit their high speed.

Most cars will often have 4 (for very old cars) – 6 gears to keep acceleration smooth and fast whilst still being efficient at high speed but trains don’t.

I get that electric motors have all of the torque available at low RPMs whilst ICE only have full power at a high rpm but wouldn’t gears still allow trains to get to higher speeds quicker?

In: Engineering

Electric cars don’t have gears, too. Electric vehicles don’t need them.
Trains accelerate slowly because of steel tires on stelle tracks. There is not much grip.

Since people aren’t strapped into a train like they are in a car and may not even be sitting down, having trains accelerate very fast would be quite uncomfortable or even dangerous for the people inside.

The problem is not the available torque in the engine. But there is a few issues with trains that make their initial departure very slow compared to cars. The first issue is that the train is very long. When the locomotive starts moving the back of the train does not immediatly start moving. As the locomotive moves forward it will start taking up the slack between the cars and also stretch out the frames of the cars a bit. Even a very tight passenger train will have a few seconds from the locomotive starts moving to the back cars starts moving. And first then will the momentum of the back cars make it lighter for the locomotive. This causes a jerking motion going back and forth through the train as it gets up to speed. In order to make this jerking motion much less which increases comfort and reduces wear on the equipment the engineers will start with the minimum of throttle and slowly increase it as they get up to speed. The second issue is that the train tracks and wheels have very low friction. This is very good for high loads and high speeds but terrible for acceleration and braking. If the engineer applies full throttle from a standstill the wheels will just slip on the smooth tracks and he will spin in place damaging both the tracks and wheels. So he have to get up to speed where he have some momentum before he can apply the full power.

Trains are awesome in that they **only need one gear**. Train tracks have much stricter limits for grade and turn radius than roads.

Trains are amazingly underpowered compared to cars. Freight trains are somewhere around 0.5 to 2 horsepower per ton. You’re average car is somewhere around 100 horsepower per ton. That’s why trains can’t go up hills more than about a 2% grade and accelerate very slowly.

However, they are very efficient. Trains can move a ton of freight over 470 miles on a single gallon of fuel.

The “torque from the start” isn’t universally true for all types of electric motors, and it’s also not entirely accurate. Many types of electric motors actually have a lot more torque available at a standstill, while others only reach their maximum torque once they spin up. Ceiling fans for example have barely any torque from a standstill, which is why they take so long to spin up and why you can easily stall them.

However, you are not wrong: For maximum acceleration, electric motors can benefit from a gearbox in order to keep the motor running in its optimum RPM range. Porsche does that with their new electric sports car. However, this adds weight and complexity, and electric trains usually have plenty of acceleration anyways.

This is going to be a very basic explanation, I understand I’m leaving out a lot of important details here.

Trains dont actually have any gears. A modern diesel electric train is essentially a generator.

Inside the locomotive, a diesel engine is coupled to a gear box. Let’s say 1:25 ratio (just an example). So for every 1 revolution of the crank (output) shaft of the diesel engine, there are 25 revolutions on the output shaft of the gearbox.

The output of the gearbox is then coupled to a generator. Electric generators and motors are essentially the same thing. Apply mechanical movement to a generator/motor and you will produce electricity. Apply electricity to a motor/generator and you will produce mechanical movement.

The power that is generated from the locomotive is used to power everything on the train. Lights, controls, heating, air compressors, and the motors that move the train.

Those motors are located under the locomotive, between each set of wheels, much the same way electric cars work.

The electricity that was generated in the locomotive is fed to the motors through a variable frequency drive (VFD). The VFD controls motor output speed by changing the frequency of the sine wave (Dont worry about this part). It kind of limits the amount of power applied to the motors.

Those motors are then connected to each set of 2 wheels through another gearbox, this time stepping down, so let’s say 25:1 ratio (again just an example). So for every 25 revolutions of the motor (input of the gearbox), there is 1 revolution of the wheel (output).

Trains are slow because they produce an insane amount of torque, and the process I outlined is what makes that torque. As others have mentioned, slow speed, high torque is exactly what trains need, as they are very heavy, and have little friction between the wheels and the rails to work with.

As they start to gain momentum, the frequency can be turned up on the VFDs to slowly add more speed, and less torque to the wheels, this is essentially what the throttle control on a locomotive does. (Again, very simplified here).

Another benefit of this is the ability for trains to use regenerative braking, the same way electric cars do. (I wont get into this),

Streetcars, subways, LRT vehicles, and some passenger/freight trains use the same process of adjusting frequency to drive motors, but instead of generating the electricity right inside the locomotive, power is fed to the train by an overhead wire, or a third rail.

it has less to do with not having the power and more about respectin g the limitations of the equipement so that they last longer and are more confortable for the cargo.

when you start moving on a train, only the locomotive has any actual propulsion that is used ot start pulling on othe other cars as it pick up the slack. this causes some stress and the frames ot expand slightly causing a bakc and forth motion. this can be minizined if the conductor slowly increases/decreases speed which in turn makes htis motion less noticeable and causes less wear and tear on the equipement.

Some trains do actually have gears. Diesel multiple units (DMUs) are often used in the UK and other countries for short passenger runs. They consist of 1-3 train cars, with diesel engines underneath. The engines are coupled to the wheels either via a mechanical gearbox (usually with automatically changing gears), a hydraulic transmission (potentially also with a gearbox), or electrically via a generator and motor.

The mechanical gearboxes are kinda rare nowadays, but there are still some trains (class 143/144 Pacers) in the UK that use them – albeit that they’re on schedule to be replaced pretty soon.

Also, smaller lighter trains can have better acceleration – underground trains in particular are pretty snappy. They’re never going to be going all that fast, so I guess the motors can be optimised for high acceleration.

Modern locomotives are electric. The diesel engine provides power to the the main generator which then goes through a rectifier and then gets sent to the traction motors (wheels) the engineer controller the speed through the use of notches or speeds 1-8. Even in 8th notch the engine is hardly over 900 rpm. 903 rpm iirc

It’s not a gear actually in the strictest sense. It does have teeth like a gear. It’s a traction motor. The electric motor turns the single gear and the teeth spin the wheel. It’s the same reason an electric car has no transmission.

A locomotive does not have a transmission in the same sense as a car. Locomotives in general are Diesel-Electric. This means that the wheels are not mechanically connected to the Engine. The wheels are connected to traction motors which provide the motive force. The motors are themselves powered by the electric power generated by the massive generators which are run by the diesel engine. The reason for this as follows.

A locomotive is typically generating about 4000-5000 HP. An IC Engine (diesel) producing this kind of power is huge including the main drive shaft. Now the wheels which are roughly the size of a truck wheel rotate at a fairly high RPM in order to move linearly at 60-90 MPH. The massive drive shaft which cannot rotate at very high speeds due to the immense mechanical forces involved. So it will need a massive transmission system to match the high speeds needed at the wheels which is not viable. This is the reason why Locomotives are Diesel-Electric. The other huge advantage of this is that it enables easy coupling of multiple locomotives to run bigger and bigger trains with one point of control. All the traction motors on the multiple locomotives run off the same electrical BUS to which all the Locomotive engine/generators are contributing electrical energy to.

The electric trains around here definitely have more than one gear – or at least something that *sounds* like gears. Perhaps the sound is something to do with switching electrical circuits for a similar effect, rather than actually changing the gear ratio, though…

I wish I could find a youtube video or something of them to share, but would you believe, people don’t seem to upload many videos of naff trains leaving stations 😛

EDIT: not a great sound recording (lots of background noise and there’s a diesel at the same time) but you can year the whine of the electric motor rising then cutting to a lower pitch and rising again several times: https://www.youtube.com/watch?v=LQu-nu3tbKU I tried counting how many “gears” the trains around here have a few times and it’s at least double figures of that sawtooth pitch thing…

Diesel trains, apart from diesel-electrics which have essentially the same transmission as AC or DC electric trains, have gearboxes. In most cases the gearboxes are fluid-filled torque converters with automatic gearchange from one or more torque ratios to direct drive ratio (equals top gear) and are known as diesel-hydraulic.

Some older trains were diesel-mechanical with manual gearchange. These were nearly all multiple-units as opposed to locomotives, and had a clutch-less planetary gearbox controlled through electro-pneumatic valves. There might be some of these left around the world, but the UK withdrew them about 15 years ago.