Saturday, May 9, 2015

Heat Kinks, or What I Learned Sitting on Amtrak for 10 Hours


After spending a few days in Toronto, I was on my way back to Syracuse via Amtrak's Maple Leaf service yesterday. We had just departed Niagara Falls after our customs inspection when our conductor informed us that, due to "heat restrictions," we would have to travel 20 mph below the usual speed limit; what's more, because of signalling malfunctions also related to the heat, we'd have to travel even slower than that for the first 10 or so miles. Because of this, we'd end up being delayed by an hour or more.

Now, frustrating as they may be, Amtrak delays are nothing new. Indeed, here in upstate New York, where Amtrak has to share tracks with gigantic freight trains that apparently always have the right of way over passenger trains, falling behind schedule is pretty much par for the course. But when your train is crawling down the track at 35 mph and you're watching cars zip by you on the highway, the whole thing just feels a little bit pathetic. (Once the signalling problems were resolved, we were able to go at 59 mph, the aforementioned 20 mph reduction from the usual 79 mph speed limit.)

But what exactly are these heat restrictions, and why are they necessary? Amtrak's own explanation is a bit lacking, so armed with an academic curiosity and several hours to kill, I took to the internet to answer this question. I ultimately found my answer at a forum called Amtrak Unlimited, which had a thread going 10 years ago on this very issue. According to the administrator of the site, him- or herself an engineer, many years ago train tracks were "jointed," meaning short stretches of track had small spaces between them to accommodate the fact that steel gets longer in hot weather and shorter in cold weather. But, as PRR60 writes:
Use of continuous welded rail (CWR) changed all that. Rail was now laid in 1/4-mile lengths and then those lengths welded together into continuous sections miles long. Physical spacing for actual expansion or contraction of the rail was no longer practical or even possible. So accommodation of the thermal effects had to take another route: stress. Push hard enough on a section of rail and it will shorthen: pull hard enough and it will lengthen. So take those miles-long sections of rail, tie them tight to the ties, anchor the ties firmly into the ballast, and ensure the ballast is heavy and well supported by the subgrade. If all that is done just right, when the rail gets hot it will not lengthen but instead will be resisted by high compressive forces (pushing), and when the rail gets cold it will not shorten but will be resisted by high tensile forces (pulling).
But when the weather is particularly hot or cold, the stress placed on the track by its inability to expand or contract can overpower these man-made forces:
If in the hot weather there is a soft spot in the track structure, particularly on a curve, the compressive force in the rails can cause the track to buckle: a heat kink. In the cold weather, the high tensile forces can find a weak spot or defect in the rail and cause a fracture. 99.99% of the time everything works as designed, but when extremes are reached, it is prudent to be cautious and increase inspections and reduce speeds to reduce track forces and lessen the potential severity of a failure.
Apparently slower trains put less stress on the tracks than do faster trains, hence the speed restrictions.

Avoiding heat kinks is a matter of public safety: the green line derailment in Washington, DC a few years ago was caused by such a kink, and Canada experienced one as well back in 2002. Not only are these heat kinks dangerous, but they also result in delays for thousands of commuters.

Which begs the question: if heat kinks are so dangerous, why put ourselves in a position where they're more likely? In other words, why switch from jointed rail to CWR in the first place? According to that same user, it's all about the maintenance costs:
Even though the installation of CWR requires some tender loving care and you have to be careful in temperature extremes, the day-to-day maintenance of well constructed CWR is much, much lower than jointed rail. Today's heavy trains just beat the track structure to death and CWR stands up much better to that abuse than jointed rail.
So CWR is pretty much here to stay, which means heat kinks and heat restrictions will continue to be part and parcel of operating rail wherever it gets hot. But are there ways of reducing the need for heat restrictions within this environment?

There's no shortage of suggestions, such as painting tracks white, hosing down the tracks when the temperature gets high (which seems like a tough sell when parts of this country are dealing with historic droughts), or changing the height of rail ties to allow for some rail expansion. MassDOT is undergoing a project to relieve some of the stress on its tracks, essentially super-heating each segment of track to test for heat kinks, and then trimming if need-be. If this is effective at reducing the frequency of heat restrictions - evidently the Framingham/Worcester line is plagued with them - then perhaps it can serve as a model for other parts of the country.

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