Why Subway Trains Screech So Loudly on Tight Curves

Eileen Engley has learned to read the platform at Boylston station the way other commuters read a departures board. "As I walk up the stairs, if I see a train coming I know to put my fingers on my ears, because I know it's going to get quite loud," she says. The sound she's bracing for is a screech loud enough that riders on Boston's Green Line have been wincing at it since the Truman administration, and it happens for a genuinely interesting reason that has nothing to do with worn-out equipment.

What's Actually Making the Noise

Trains don't have differentials — the mechanical linkage that lets a car's two front wheels spin at different speeds through a turn. On a train axle, both wheels are locked to spin at exactly the same rate. Going around a curve, the outer wheel needs to cover more distance than the inner one in the same amount of time, and with no differential to absorb that difference, something has to give: one or both wheels slip sideways across the railhead.

"Think of the wheel as a circular bell," says Jeff Zapfe, president of the Cambridge acoustics firm Acentech, which has studied the Green Line's noise. When the wheel wants to go straight but the rail forces it into the curve, the mismatch sets off a stick-slip motion — the same family of friction physics that shows up in everything from squeaky door hinges to the stress that triggers earthquake swarms. It's almost like bowing a violin string, Zapfe says — the wheel starts vibrating, and because a train wheel happens to be an excellent radiator of sound, that vibration comes out as a piercing, high-pitched squeal rather than a dull scrape.

Why Some Curves Are Much Worse Than Others

The sharper the curve, the more slip, the louder the squeal — which is exactly why Boylston has become the signature offender on the whole system. The Boylston curve, I understand, is one of the sharpest turns in North America, Zapfe says. Inbound Green Line trains make close to a 90-degree turn from Boylston Street onto Tremont, through a tunnel that opened in 1897 alongside neighboring Park Street — the first two subway stations built in North America. At one point, the resulting screech at Boylston has been measured around 111.3 decibels, comparable to a chainsaw at close range, and researchers studying curve squeal generally have clocked the phenomenon as high as 120 decibels on the worst offenders worldwide.

Kimberly Woollard, the MBTA's deputy director of light rail vehicle maintenance and engineering, put the constraint bluntly: You can't really change the radius of the curve because the tunnel is so old. It's such a tight, tight corner. It was made for trains that don't exist anymore. More than a century of Boston's subway infrastructure was built around vehicles nobody runs anymore, and short of rebuilding the tunnel, the geometry simply is what it is.

Squealing Isn't the Only Noise a Train Makes

Curve squeal gets the attention, but transit engineers hear a train's whole vocabulary. Henry Kolesar, BART's chief vehicle engineer, told KQED his system makes three distinct sounds passengers notice: a rising electronic hum during regenerative braking, which is actually the train converting motion back into usable electricity; a low, buzzing groan from wheels rolling over "corrugated" rail that's developed bumps over years of use; and the high squeal on curves. The trains make quite a few different sounds. Some of them are music to the ears of engineers, Kolesar says — the braking hum, in particular, is the sound of a system saving power, even if it doesn't sound that way from the platform.

Can Anything Actually Fix It?

Partially. The MBTA runs 13 grease-dispensing units along the Green Line, with six more added over the years, that pump a small amount of lubricant onto the wheels and rail as trains pass. More recently, engineers finished installing flange stick lubricators on the newer trains — a mechanism Woollard compares to a giant Pez dispenser, pressing a stick of graphite against the wheel as it turns. A separate fix used on squealing curves elsewhere involves bonding rubber dampers or tuned metal absorbers directly to the wheel, muting the vibration before it can radiate as sound the way a mute quiets a trumpet.

None of it eliminates the noise entirely, and lubrication comes with its own tradeoff: too much, and the wheel loses the grip it needs to actually turn and stop safely, so crews apply it sparingly and monitor how well it's working. Engineers researching the phenomenon note that squeal severity shifts with humidity, temperature, train speed and how worn the wheel profile has become, which is part of why the exact same curve can shriek on a dry August afternoon and stay nearly silent the next morning after rain.

Why Doesn't Every Curve Squeal the Same Way?

Because so many variables stack on top of each other — wheel diameter, whether the axle is powered or coasting, how recently the rail was ground smooth, even the slant built into the rail itself. BART listeners have compared their system's version of the sound to a baby seal calling for its mother and a slowly deflating balloon; Boston's version has been irritating commuters since before most of them were born. Both systems are chasing the same fix on the same century-old physics problem, one graphite stick and one greaser unit at a time — a reminder that plenty of the noises and quirks built into daily infrastructure, like why a lake flips itself over every autumn, come down to physics nobody bothers to explain until it gets loud enough to notice.