Flat junctions are problematic enough on the London Underground. They're a real mess on the south London rail network. Don't know Boston's commuter rail that well - is it that bad? Or is it worse because passenger rail hasn't been the priority it has been elsewhere?
But it's a difference that only came about by historical accident, and even then only partially (as you've described, there *are* through services in London, just very little integration between suburban and urban lines).
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Paris is more clearly segregated. But you usually don't want 300 metre commuter trains on the same tracks as 130 metre subway trains
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Correction: you *can't* have them on the same platform. If they were of a compatible length, why wouldn't you want them to? Moreover, why should they be of such radically different lengths in the first place?
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Mainly the cost of building, and especially extending, underground stations. And the time taken to clear stations and points.
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I'm not sure you read my tweet: *if they were of compatible length,* why wouldn't you want them to use the same tracks? And, following on that, what advantage is conferred to the riders by having them of such different lengths in the first place?
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Bear in mind that if you want to double capacity on a line, you can either double the size of the trains, or double the frequency: the latter option has the extra benefit of shortening wait times for riders and increasing the number of possible trips in a day.
End of conversation
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