Why does the electricity shut off on my train just before exiting or entering a station?
December 17, 2010 7:41 PM   Subscribe

Every time I take the train, just before leaving or entering the station, all the lights go out and it seems like the ventilation shuts off too. A couple minutes later everything comes on again. Why does it do that?
posted by Sully to Travel & Transportation (12 answers total) 3 users marked this as a favorite
Wait--the lights and ventilation are off for "a couple minutes"? I've seen them flash for a few seconds, but never in the range of minutes. What train are you talking about?
posted by MrMoonPie at 7:46 PM on December 17, 2010

Best answer: Sounds like you're on an electric train and it's passing a phase break or neutral section. Parts of the overhead wires (or the third rail) are left un-electrified in order to break the network up into manageable sectors (and to avoid some complicated electrical problems which I admit don't quite understand). When the train passes by one of these, it loses power, and the lights and ventilation go off.
posted by embrangled at 8:00 PM on December 17, 2010

This happens on the San Francisco muni when going underground (or when going aboveground); in that case, my understanding is that this is when the system switches from automatic (underground) to manual (aboveground) control. It's usually just for a moment or two. This also happens on NJ Transit when going into Secaucus Station, and I always assumed there was a similar reason related to switching power sources or control systems.
posted by dreamyshade at 8:02 PM on December 17, 2010

Response by poster: MrMoonPie, I'm on the ViaRail here in Canada, coming out of Montreal and going to Toronto.
Embrangled - that sounds about right, what they said about coasting to the next electrified section. There is definitely that sensation.
posted by Sully at 8:34 PM on December 17, 2010

The via is still diesel, afaik. I'd imagine they cut the engines if they are going to be sitting in Union Station or something, and the engines on idle likely can't power all the lights for the train. That's just my uneducated guess.
posted by glip at 9:02 PM on December 17, 2010

Via is not and electric train.
posted by saradarlin at 9:31 PM on December 17, 2010

I know exactly what you are referring to. The Via train leaving Montreal seems to have a fairly long period when the lights are out and the ventilation isn't running (I wouldn't say it is a couple minutes, but it certainly isn't momentary). I always assumed it was switching from the electrical source used while in the station (batteries or possibly a plug-in system) to power from the engines.

If I recall correctly, this only happens on the Montreal end of a Montreal-Ottawa trip, so I assume it has something to do with the train being inside and so they aren't running the engines (the train stays outside in Ottawa).

Why does the switch take so long though? I think this is one of the many mysteries of Via.
posted by ssg at 11:37 PM on December 17, 2010

I've experienced exactly the same thing, and have heard it explained by an announcer as the train being detached from station power and connected to the main rail power. The innards of Montreal's Central Station are not new, and this is probably simply done by hand.
posted by zadcat at 7:41 AM on December 18, 2010

In the old days passenger cars had individual electric generators powered by their own small engines or even connected to the axle of the wheels. Today they are equipped with what is called head-end power (HEP). This means that the electricity for the whole train is generated by the engine at the front of the train and passed by large electrical cables from car to car to the last car in the train.

In most cases the head-end electric generator runs off the same diesel engine that runs the traction motors that pull the train. The cars require a lot of power to run the lights, air conditioning, heaters, stoves and refrigerators, called hotel power. The full train might need 500 to 700 kilowatts of electricity which is supplied as 480 volts three-phase. This is the equivalent of 700 to 1000 horsepower. Depending on type, the train engine is capable of 3000 to 4000 horsepower total to pull the train and supply electricity.

To get the train started it needs full power directed to the traction motors so by turning off the electrical generator temporarily it gains an extra 1000 horsepower or so. The engine has three modes -- traction power only, HEP only and combined HEP/traction. The engine turns at different fixed speeds for these three modes so there are few seconds when switching between modes while the engine adjusts and the switch gear operates that the electricity will be off. While in the station it will be in HEP only mode to supply power to the cars. When it leaves it will be in traction only mode and then switch to HEP/traction mode. When it enters a station it switches from HEP/traction mode to HEP only mode. This switching between modes is the reason the lights go out briefly.
posted by JackFlash at 8:53 AM on December 18, 2010 [4 favorites]

Fascinating. And it might be more noticeable at Central Station because the parked trains are inside a dark shed and when the lights go out, you can't ignore it.
posted by zadcat at 11:42 AM on December 18, 2010

I've occasionally noticed this same phenomenon on the Metra trains in Chicago, and they're indeed diesel. The November issue of the Metra's newsletter gives this explanation:

When the train is in the station, the lights and other systems are powered by plugging the trains into the electricity at the station. That allows us to idle the engine to save fuel and limit emissions. When they leave, the engine supplies the power. So when the lights go out, it’s because we’re switching from the plugged-in power to the train. Sometimes that’s pretty seamless so you don’t notice. Sometimes it takes few moments.

Or, what zadcat said.
posted by gueneverey at 2:16 PM on December 18, 2010

JackFlash has it. The Quebec City - Windsor corridor isn't electrified; VIA runs P42DC Genesis and F40PH-2 diesel-electric locomotives on it. The notorious designs of the HEP systems of both of those locomotives means that they have to lock the diesel engine at 900 rpm to power the cars, even if the train is sitting idle at the platform. Because the HEP saps power from the traction motors at a rate of 1 horsepower per 0.746 kw (1072 hp for the P42 and 670 hp for the F40) and because the engines can run past 900 rpm, cutting the juice is a viable option to get up a steep grade or to get up to speed quickly.

VIA is currently in the process of rebuilding their fleet of F40PH locomotives. One of the improvements is installing a separate engine solely for HEP, so this might become a much rarer event.

Interestingly, Metra also runs the F40PH, in several variations. I've never heard of a diesel-electric trainset being plugged in at a station, though.
posted by clorox at 12:39 AM on December 20, 2010

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