Source of rising locomotive sound?

July 17, 2009 9:51 AM

What is the MMM ^MMM!sound an idling locomotive makes every few minutes?

I used to live by a rail yard, and recently took a long train ride from Boston to Chicago. I've noticed a weird rising sound that idling locomotives make occasionally. It's very loud, has a rising pitch, lasts a second or two, and is so unusual and distinctive that it sounds like God's own spaceship being charged up. What is it?

posted by Mapes to technology (5 comments total)

Probably the turbocharger, I would think.
posted by le morte de bea arthur at 9:57 AM on July 17

..assuming it sounds something like this.
posted by le morte de bea arthur at 9:59 AM on July 17

I think I know the sound you mean, and I think it comes from a specific type of air compressor, used to supply pressurized air for the brakes and other things. Not all locomotives make that sound, though they pretty much all do have air compressors on them. This air gets used (and leaks) constantly, so the compressor has to cut in and out every few minutes to keep the main reservoir topped up.
posted by FishBike at 10:07 AM on July 17

Ah, I should have used YouTube from the beginning. Here are samples: 1, 2, 3.
posted by Mapes at 4:10 PM on July 17

Now I'm sure the sound you're hearing is the start-up of electrically driven compressors. Each of those videos seems to start with that sound, like a big electric motor starting up (which is what it is).

Note in the last video where there are two locomotives coupled together, it sounds like there are two sounds in rapid succession? That's because when one compressor starts, they all do, so that multiple locomotives can supply more compressed air than if just one was trying to do it all.
posted by FishBike at 4:23 PM on July 17