General Question

wildpotato's avatar

Why do I hear low, metallic twanging as trains arrive at the subway platform?

Asked by wildpotato (15224points) August 21st, 2009

I mean the MTA subway, in New York. If you’re standing by the very end of the tunnel where the train is going to emerge from, it’s really noticeable – dozens of low-tone twangs, overlapping. You can hear them echoing even before you can see the train’s lights, sometimes. What is this sound? If I had to guess, I’d say it might be something to do with the way it moved on the rails, but everything I picture (like trains juddering ever so slightly in and out of contact with the rails) seems implausible. Any thoughts?

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21 Answers

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XOIIO's avatar

lol @pdworkin

i would say it has to do with the vibrations created by the train moving, this can cause pipes and beams to rattle, bend and shake. it sounds distorted and they overlap because the tunnel is a great echo chamber, and sound carries a long way. hope i helped.

PerryDolia's avatar

I would guess that the rails are moving slightly. As a wheel rolls down a rail and reaches the end, the other end is lifted slightly (like a teeter totter). One of the wheels behind hits this slightly raised end of the rail and causes the twang.

Grisaille's avatar

I think it’s the wheels, as the come to a stop. Because an object of that size has massive kinetic energy, when you abruptly hit the brakes and stop the wheels, the train still moves. What you (well, we. I live in NYC, too) could be hearing are the wheels turning and “breaking the brake”, so to speak. I imagine the brake cannot handle such force, and are designed to “click”, moving forward a fraction of a rotation as the train is coming to a stop. If you were to make the wheels completely immobile whenever the brake is hit… uh… something bad might happen. I don’t know.


XOIIO's avatar

awww why did they remove pdworkins comment? it didnt seem offensive. although i guess the none of your buisness part. oh well.

BTW wildpotato i feel sorry for you, considering you have nothing better to do than count echos.

wildpotato's avatar

@pdworkin Zombie announcements never occurred to me. GA.

@XOIIO They removed it because you aren’t supposed to joke until the second answer in. And it’s just something I was wondering one day, looked up and couldn’t find the answer to.

syz's avatar

[mod says] The guidelines clearly state that joking and “off the cuff” responses should not occur at the beginning of a thread as they tend to derail the entire discussion. Humorous responses, if they occur, should be later in the thread after a questioner has received serious replies.

dpworkin's avatar

They removed it because it wasn’t a serious answer. If I had posted it now, it probably would have been OK. Take a look at the Guidelines. Sometimes you get away with a joke, sometimes you don’t.

XOIIO's avatar

ok, it makes sense that a joke should be later on.

Noel_S_Leitmotiv's avatar

It’s the sound of the pickups sliding along the electrified rail.

Deepness's avatar

Trains and sounds. Yay! A question I know plenty about.

You could be hearing several things.

Each rail is 39 feet long. They are welded together. It’s possible you are hearing the sound of the wheels passing over the the joint.

Also, the wheels on trains are not perfectly round. Due to braking, some of the wheels may develop what’s called a “flat” where it basically has a slight flat side. That’s what gives trains the sound of thu-thum thu-thum thu-thum.

There are plenty of other sounds to a train but these are the two most likely to answer the question.

dpworkin's avatar

Oh. I forgot. ObTopic: Zombie announcements. None of your business.

Deepness's avatar

@XOIIO Your answer is very true also.

Deepness's avatar

@Grisaille Trains actually use brake pads similar to cars but much larger. One set of brake pads for each wheel.

Grisaille's avatar

deepness be schoolin’ us on MTA matters, yo

wildpotato's avatar

Huh, well now I’m even more puzzled – what Deepness said makes sense, but when I used the PATH earlier today, the twanging didn’t happen. Are those things you said about the MTA system not true of the PATH system?

Deepness's avatar

@wildpotato I know lots about MTA subway. I’m not familiar with PATH or LIRR or Metro-North, or any other railroad for that matter. There’s an infinite amount variables between all railroads that are not just limited to trains and rails but also include procedures, rules, regulations, system, safety, etc.

wildpotato's avatar

@Deepness Gotcha. Thanks!

wilbert's avatar

this is normal, as the wheels are scraping against the metal rails. like regular trains. They tried to reduce this in Montreal Que. by adapting rubber to wheels, all it did was heat up rubber and made subways so hot, you could faint from the heat given off by tires. In a way, hated when trains arrived, but happy to get in. Now they must use more A/C in order to tolerate heart from wheels

Deepness's avatar

@wilbert I don’t think the question is referring to wheels screeching. Rubber parts were adapted to the wheels here in NYC also to reduce the screeching noise. The question above is asking about a “low metallic twanging” sound.

Deepness's avatar

@Noel_S_Leitmotiv WHat you refer to as “the pickups” is actually called a contact shoe. There are 6 shoes per train car on IRT lines and 8 shoes per car on BMT lines here in NYC. These shoes slide along the third (electrified) rail and provide power to the train. They really don’t make any noticeable noise except for the occasional arc of electricity they generate which is a crackle sound.

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