Epistemic status: Main part is well-supported but may have some minor errors. The parts about potential future lines are inherently speculative.
Small Train is Good Train
A while ago, I wrote about how elevated trains are the greatest urbanism cheat code, increasing the amount of track miles you can build per dollar (or per year) by a factor of 2-4. And while I don’t have anything else on that order of magnitude, I do have one more easy 20-50% gain: Run shorter trains.
The basic idea is simple: The single biggest cost of any metro system is the stations, whose cost scales with size. Therefore, if we run a system for smaller trains, we can build smaller stations for these trains, saving a huge amount on station costs. This costs us in reduced total capacity, but this can easily be made up for by increasing train frequency.
This also has some non-obvious advantages: Smaller systems let us have smaller, more incremental transit projects, gaining project experience and institutional knowledge for subsequent projects. Especially in places like the US, which don’t have recent experience in building infrastructure, it’s better to start small where possible.
It’s also just better to split up your capacity as much as reasonably possible. Having to wait for a train that comes every three minutes is a lot better than waiting for an equally-crowded train that comes every six minutes. Wait times are the second-most underrated component in making metro systems effectively faster (right after street to platform time).
Some details
Cost and Timeline improvements
Stations usually dominate metro construction costs (23 – 62 % of hard cost on the Transitcosts European projects sample, mostly depending on underground vs elevated) and can be made roughly half as long (50-55 meter platforms vs ~110 meters) for low-capacity short trains. Station construction costs scale roughly with station size, so cutting station size in half implies a total cost reduction of roughly 20% on underground lines and 15% on elevated lines.
Timeline improvements are even sharper. Since the stations are the single biggest (and thus longest to build) component of the system, anything that makes them easier to build makes the whole system open faster. You can’t quite build a station twice as fast if it’s twice as small, but you can typically hit 30-40% timeline reduction (3-4 years in absolute terms) on a medium-sized project (e.g. a 10-15km subway line) where stations are on the critical path.
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