How to keep a constant speed

[Charlie B]

You click the power on and off, sometimes all the way off. You can also use a little train brake, not good for freight as it wastes fuel, and from what I hear is not allowed anymore, but on a passenger train it stretches the slack and gives a smooth ride.
You may not realize it but your foot on the throttle in a car is never steady, you are always increasing and decreasing the pressure.
Charlie