In French, there is a word erre which is the residual speed of a train, a ship or a car (or whatever is moving and needs propulsion).
For example, if you see a red light in your car, you stop accelerating, but you don't brake either; you just let the car move on its own. You can say that your car is on erre.
For a ship, if you cut the propulsion of the engine, you are on erre.
In a train, it is used if there is a switch of electrical alimentation. For a hundred meters, the train is no longer fed with electric power, and it continues moving with its residual speed. The train is on erre.
What is the best English expression for this?
Best Answer
For a car or a train, if you stop using the engine to propel it, you can say that you coast to a stop, or that the train (or car) is coasting. I haven't heard this used for boats (and Googling seems to indicate that if you coast in a boat, it often means that you are following the coastline), but I don't know what term would be used instead.
UPDATE: As Zairja remarks in the comments, for a boat the corresponding verb is drift.