Another good reason to pile density onto subway lines: Almost free heat and cooling.

A few years ago we noted a good reason to take the subway: It's warmer below. Then-Mayor Boris Johnson, who is also full of hot air, described how they would heat 700 homes. Now researchers at L'Ecole Polytechnique Fédérale de Lausanne (EPFL) have calculated that they can recover that heat, which comes from brakes, motors, people and just the warmth of the ground in general, and move it with heat pumps.

The system works in a similar way to a refrigerator, with plastic pipes containing heat-transfer fluid, or simply water, placed at regular intervals inside the concrete tunnel walls and connected to a heat pump. In winter, cold water will be pumped into the pipes, emerging hot at the surface. The opposite will happen in summer. According to the researchers, the system would be cheap and energy-efficient to install and would have a lifespan of between 50 and 100 years, with only the heat pumps having to be replaced every 25 years.

Margaux Peltier, whose masters thesis is the basis for the study, calculates that if they line half the new Lausanne M3 subway with heat recovery pipes, they can heat 1500 standard 800 SF apartments, "or as many as 4,000 Minergie-certified energy-efficient units." Minergie is sort of a Swiss version of Passivhaus. “Switching from gas-fired heating would cut the city’s CO2 emissions by two million tons per year,” adds Peltier.

The research is yet another example of why you can never separate land use and transportation. In most cities, subways are built to service high densities, which is where district heating systems work best. So if you build ultra-efficient medium to high density housing on top of a subway system, you not only can most of the heating and cooling of air and domestic hot water with the heat pump, but you can also move the people without cars, saving many million more of tons of CO2. What a wonderful idea.