system
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human
ELI5: 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?
gpt
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.