I know, up north the Senate is debating ... uh ... something that might impoverish and sicken millions of people, while the President burns through every norm we have left. But I am south of the border, so let’s talk about subways.
Back in 2014, we blogged about the Line 12 mess. (The above station is on Line 12 where my boy goes for soccer practice.) The problem was that the city decided to use steel rails and overhead catenary power lines instead of rubber tires and third rails. The trains proved too heavy for the tracks, 15 kilometers of which had to be replaced. In addition, curves on the elevated structure had to be widened and the camber reduced from 160 millimeters to 100. It was a design mess, but as I argued in the post, the Mexican government actually comported itself fairly well in the aftermath.
The question is, why did they decide to switch designs? This wasn’t the first steel-wheeled line constructed in Mexico City, but Line A runs out the eastern suburbs mostly on a straight ground-level right-of-way in the middle of a freeway. Moreover, Line A was originally designed to be a light rail that would link into the end of the Metro but not really be part of it. Its cars are a full 30 centimeters narrower than regular metro cars and weigh 88 tons less. (The history of Line A can be found here.) In other words, the decision to build Line 12 with overhead wires and steel rails had nothing do to with the earlier experience with Line A.
The decision seems to have been based on a 2000 study for the World Bank, which compared Madrid’s successful subway roll-out to experiences in Mexico, Caracas, and Santiago. Madrid used steel and overhead and got it done cheaper. The reason was simply that most of its lines had historically used overhead wires and its maintenance operations were designed around them. (You don’t have to turn off the power to protect maintenance crews when you have overhead lines.) So it could use the same sort of railcar that it always used. In that sense, Madrid went with overhead for pretty much the same reason that Mexico City should have stuck with third rail.
As for steel versus rubber, the study provided evidence that steel wheels should be cheaper. Steel wheels last longer, so there will be some savings over the long-run. And you need three sets of rails for rubber-wheeled trains, because you need guide tracks to keep them on course. So steel should be cheaper. But is it? As the study itself noted (page 77), Mexico’s rubber-wheeled trains can operate on 7% slopes and make turns with 105 meter radii, compared to 4% and 250 meters in Madrid. If Line 12 had used that technology, it would not have needed to be rebuilt.
In short, it looks like the Line 12 decision was the triumph of hope over experience. There were plenty of reasons to believe that steel wheels and overhead wires would be a better design if you were starting a subway from scratch. But Mexico City was not starting a subway from scratch. It was extending a well-established system. And its only experience with steel-wheeled cars was on what was a suburban light rail with much smaller cars and trains than the rest of the system.
Still, the ending so far has not been unhappy. As we noted in 2014, the cost of the new line is still reasonable. Moreover, in May 2017 the government obtained a further discount of 2.1 billion pesos from the consortium built the line. And ...
... the new trains, they are very nice!
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