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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?

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what is a switch of electrical alimentation? –  TimLymington Oct 2 '12 at 16:02
    
alimentation in French means supply. –  Peter Shor Oct 2 '12 at 16:26
    
Is it much used? It is in neither my Collins bilingual dictionary, nor in my Larousse Dictionnaire du français d'aujourd'hui. –  Barrie England Oct 2 '12 at 16:27
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@BarrieEngland Try this –  StoneyB Oct 2 '12 at 18:37
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It is definitely not a used expression. I think 99% of french people doesn't know it's existence! –  Cyril Gandon Oct 2 '12 at 21:59
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4 Answers

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.

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I think in English, for cars and trains, we would use the word coast both for the expression courir sur son erre and for putting the gear in neutral. I don't sail, so I don't know whether the same word is used for boats. Maybe you're looking for the word inertia. –  Peter Shor Oct 2 '12 at 15:38
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When this occurs on a boat, you can call it drifting or say the boat is adrift ("without motive power and without anchor or mooring"). –  Zairja Oct 2 '12 at 15:53
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Coasting in a car does not imply that the gear is in a neutral position. It only means that there is no propulsion. If you let your foot off the accelerator, you are coasting. –  MετάEd Oct 2 '12 at 16:20
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In my mind, a boat that is drifting or adrift is likely not moving at all. I don't really know of a better word for a boat, though, except something more technical like momentum or inertia. (Though inertia could, again, apply equally to something that isn't moving.) –  Marthaª Oct 2 '12 at 18:57
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Google has around the same number of hits for boat drift to a stop and boat coast to a stop, so I think drift works. I don't think adrift does, though; that seems to me to indicate that the boat is at the mercy of the currents and wind. –  Peter Shor Oct 2 '12 at 21:57
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I think the word you are looking for here is momentum, as in traveling on momentum alone.

momentum n
2. (Physics / General Physics) the impetus of a body resulting from its motion

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And the idiom is the unusually long "travelling under its own momentum". "Coasting" saves a lot of keystrokes. –  StoneyB Oct 2 '12 at 17:24
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@StoneyB: The OP was looking for an equivalent to on erre. The question was not about how to save keystrokes. –  Robusto Oct 2 '12 at 17:27
    
@StoneyB The car "continued on momentum" versus "was on erre" or "coasted" isn't that much longer. You could also say something "pushed/moved/kept/went on/by momentum". –  Zairja Oct 2 '12 at 18:22
    
@Robusto Sorry, I misread your answer as offering "momentum" by itself as the answer. Haste makes embarrassment. –  StoneyB Oct 2 '12 at 18:23
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You can also consider freewheel:

  1. (of a gear) To continue spinning after disengagement.
  2. (of a cyclist) To ride a bicycle without pedalling, e.g. downhill.
  3. (of a motorist) To operate a motor vehicle which is coasting without power, e.g. downhill.
  4. (by extension) To operate free from constraints.

Wikipedia has a page devoted to this subject:

The condition of a driven shaft spinning faster than its driveshaft exists in most bicycles when the rider holds his or her feet still, no longer pushing the pedals. In a fixed-gear bicycle, without a freewheel, the rear wheel would drive the pedals around.

An analogous condition exists in an automobile with a manual transmission going down hill or any situation where the driver takes his foot off the gas pedal, closing the throttle; the wheels want to drive the engine, possibly at a higher RPM.

P.S. The word is more often used in its figurative sense.

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The train is 'traveling under its own inertia':

Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion.

The train is resisting the frictional forces acting on it, and thus slows, but continues without an external driving force acting on it.

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I don't speak much French, but I do know that the word inertie (= inertia) also exists in French. But perhaps inertie has a different nuance of meaning from inertia in non-technical usage? –  Pitarou Oct 2 '12 at 23:17
    
Argh, “it’s” => “its”. –  Konrad Rudolph Oct 3 '12 at 10:34
    
@KonradRudolph mischief managed –  Pureferret Oct 3 '12 at 10:45
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I like this one, simple, can be use in french too, and have the exact same meaning. We can said "the train is on his inertia" as well as "le train est sur son inertie". –  Cyril Gandon Oct 3 '12 at 13:03
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