Ask Engine Bob

Q: Engine Bob, is it legit for Metro North to blame late trains on snow and ice? I mean, how many problems can a few inches of snow really cause a train?

A: Believe it or not, quite a few. Back in the 1950s, the Lackawanna Railroad (and, no doubt, some others) struck on a way to gain an advertising edge over the airlines by claiming that a trip by train was “weatherproof.” Um, nice try. While obviously a huge snowfall can cause problems for any means of conveyance, the fact is that even small amounts of snow and ice—especially when outside temperatures remain below freezing for extended periods—can bring a train to its knees (or its couplers, whatever).

First, I present you the easy ways that snow and ice can botch up a train. Even a thin layer of snow on a rail reduces the train’s adhesion coefficient—er, just think traction. On slick rails, the train wheels will slip and even skid considerable distances. Bad thing.

Next, snow, ice, and below-zero temperatures can also delay or even disable trains by freezing up the switches that connect one set of rails to another. If several or even one important switch is stuck, trains cannot be routed to the right lines. So they have to sit there.

Until when—the spring thaw? Well, no. Many railroads—Metro North included—have actual heaters installed at key switch points to keep ice from clogging up the works. But when it’s really, really cold outside, even switch heaters don’t always work. (I am not kidding about what I will now relate because I have actually done it myself: Sometimes the only way to free an iced-up switch is to take a flamethrower to it.)

Okay, those are the simple ways that ice and snow cause train delays—but they are not the main culprits. Those come next, and they plague Metro North in particular because the majority of its trains run on externally-fed AC power. The high voltage required to power a train is delivered to the traction motors via either the third rail—that’s the raised, thicker rail you see just off to either side of the track—or by a catenary system, which is the suspended, high-tension wire that looks as though it’s part of a spider web strung along the line, several feet above the tops of the train cars. (You’ll find third rail on the Hudson and Harlem Divisions; catenary on the New Haven. The diesel engines are used in territory that is not electrified, such as the Hudson tracks north of Croton-Harmon and branch lines of the
New Haven like Waterbury.)

When snow piles up on the third rail, it can seriously interfere with the mechanism that delivers the current to the car’s traction motors. If you’ve ever watched trains pass from a vantage point on a platform, have you ever noticed those square, flat pieces of metal that stick out about a foot from the passing train’s wheel trucks? Those are called “shoe plates,” and they’re designed to slip along the top of the third rail and induct electrical current in order to power the train.

However, when snow accumulates in patches on the third rail, the shoe plates start skipping. When that happens, the high voltage from the third rail “arcs”—literally, leaps through the air—to get to the shoe plates. Arcing’s searing temperatures will melt the snow on the third rail. That melt will re-freeze into ice after the train has passed. And then the ice that has resulted (now acting as an insulator) will only further interfere with the shoe plates’ ability to draw voltage when the next train rolls along. Eventually, the connection gets so bad that the traction motors on the train don’t get enough juice. The motors stall, the train stops, and that’s the end of that.

A similar process takes place with the overhead catenary wire. Ice will form along that, too, and sometimes get so thick it can effectively insulate the wire so that no voltage can reach the train. Atop the roofs of New Haven cars, you’ll see those Z-shaped contraptions (they look to me like huge disposable shaving razors) that can be extended upwards and retracted back down. These are called “pantographs,” and they are the conduit for power between the overhead wire and the train. An ice-covered catenary wire keeps voltage from the pantograph just like an iced-over third rail keeps voltage from a shoe plate—same exact principle. Same exact result, too: Arcing between wire and pantograph melts snow; snow re-freezes into ice; ice keeps voltage from reaching the train; motors stall; train stops.

Oh sorry, I forgot the last step: …train stops, commuters pissed.

An icy wire can also rip up the carbon strips on the catenary’s induction surfaces, eventually rendering them useless. Ice will also form on the pantographs themselves, disabling them so they can’t even be extended up to the overhead wire at all. At times, even the weight of the ice clinging to the overhead wires will bring the whole catenary web down to the ground.

Tip: Should you see this happen, ever, do NOT try to help by picking the fallen wires up; you’ll be roasted like a partridge.

And your charred, smoking carcass will become yet another reason for a train delay.

—Engine Bob


Got a question for Engine Bob? Hit him up here:  

This entry was posted in Ask Engine Bob, Catenary, Engine Bob, Metro North, Snow. Bookmark the permalink.

1 Response to Ask Engine Bob

  1. Pingback: Commuting on New Haven Line Back to Merely Irritating « Trainjotting

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