Answer Man is just one of the many corpuscles that travel every day in our city's great subterranean vascular system: the Metro. Sometimes as he waits for his train, he wonders. . . .

Is there anything more infuriating than standing on a crowded platform and watching an empty train go past?

The answer, of course, is no. But why do empty cars sometimes breeze by? Shouldn't Metro cars always be filled with Metro riders?

Metro spokesman Steven Taubenkibel said there are several reasons a honking train marked "NO PASSENGERS" might travel through a station. The train might have been removed from service because of a mechanical problem. An empty train might be transferring from a service and inspection shop to a repair shop to be fixed.

The empty train could be on its way to rescue another train that broke down, to pick up that train's passengers. It might have been called to push another train that can't move on its own. Or the train might have been removed from service because of a sick passenger. ("On occasion," wrote Steven in an e-mail to us, "we have to remove trains from service in the event a customer becomes ill, requiring the train to be cleaned immediately." Ewww. Take your time.)

Finally, Steven said, the empty train could be what's known as a "gap train," one that's being put into position to be available just in case another train breaks down.

How come you get so many blasted nickels in your change when you buy a Farecard?

Has this ever happened to you: You stick a $5 bill into a Farecard machine to buy a card worth $2.90, and it suddenly sounds as if you hit the jackpot at the nickel slots. You scoop dozens of the fat little coins into your pocket then spend the rest of the day jingling around the office.

That shouldn't happen, said Robert Glenn, Metro's supervisor of fare collection engineering. Any time you're due five nickels, you should get a quarter. Here's how it's supposed to work: Let's say you're buying a $1.35 farecard with $2. The machine takes the amount you're due in change -- 65 cents -- and converts it to nickels. That's 13. Then it divides that by 5, to figure out how many quarters it can give you. It would come up with 2.6. It should give you two quarters and then sixth-tenths of a quarter: 15 cents, or three nickels.

What it won't ever give you is a dime.

The coin-dispensing hoppers that Metro uses can be set up to spit out either nickels or quarters. Each machine has three hoppers: two set up to dispense nickels and one to dispense quarters.

"That's all we give: nickels and quarters, since all our fares are based on a nickel increment," Robert said. The thin little 10-cent pieces are hard for the Farecard machine's hoppers to deal with. Also, the nickel/quarter hoppers are similar to one another and can be easily modified to take one or the other.

I swear, though, that I've received six or more nickels in my change before. Robert allowed as how that sometimes happens, but only when the quarter hopper is empty or "dead." All 394 regular Farecard machines, 240 Exitfare machines and 230 debit card-accepting "express" units can be monitored remotely to see when they need topping up.

Why does it sometimes get deathly quiet when a train is stopped between stations?

It's really eerie. The heating or AC goes off, people stop talking and it gets as silent as a crypt.

The reason is electricity, or lack thereof. Every Metrorail car has a "collector shoe" that connects to the electrified third rail to provide power. Throughout the 103-mile rail system, there are gaps in the third rail. Depending on where a train stops -- between stations or traveling through a switch (or interlocking) -- the collector shoe may not touch the third rail.

The heating ventilation/air conditioning units click off until the train starts to move, and the collector shoe connects again with the juice.

So that's why the train's mechanism gets quiet. Why do the train's passengers get quiet?

"One of the things that's consistently shown is that noise tends to agitate a crowd," said Ryland C. Gaskins III, a crowd control expert with the Virginia Modeling, Analysis and Simulation Center in Suffolk. "So the opposite is true: Take away the noise, and they're going to calm down."

Ryland said this is probably a vestige of the evolution of humans, where sudden silence was a signal to be more aware.

What are those boxes marked "Stinger" that hang from the side of some Metro tunnels? They're not antiaircraft missiles, are they?

They are not antiaircraft missiles. Think of them more like gigantic jumper cables.

"It's standby emergency power," said Steven. "They're not everywhere, just located at double crossovers or interlockings where trains can switch from one rail to another."

If a train loses power, Metro personnel can go out in the tunnel and set things right. "The Stingers basically are cables," said Steven, "and you have to take one end of the cable and clamp it to the third rail, and you clamp the other part to the collector shoe under the rail car."

Remember to wear rubber-soled shoes.

Doors Opening

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