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MortimerAdler t1_j60mp5t wrote

How do they measure this with stations being different distances apart?

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pm_me_good_usernames t1_j60ufjr wrote

If a train leaves the end of the line 12 minutes after the previous one, and they go at the same speed and spend the same amount of time stopped at each station, then it should get to each station 12 minutes after the other one did no matter how long the line is or how far apart the stations are. Of course that's a lot of ifs, so these numbers are more of an average than anything. But they do try to keep them as consistent as they can--if you've ever been on a train that stayed at a station for several minutes because of "schedule adjustment" that means it had been going faster than the other trains on the line and needed to slow down to try to keep the timing consistent.

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Capitol_Limited t1_j611pwf wrote

To piggyback on this great explanation, this is why the # of train cars/train sets is important, and why losing the 7000 series was huge. Not every line needs the same number of trains, because as the original asker stated, the distance between stations (and on each line) is variable.

For example, it takes about 102 mins for a green line train to complete a roundtrip, not including layover. For service every 8 mins, it would need about 12-13 trains to maintain that.

By comparison, it takes 186 mins to do a roundtrip (again, not counting layover) on the Silver line. For every 15 min service, though, which is nearly double the green line, you still need about 12-13 trains. If the silver line were to do 8 min headways (which I don’t think is possible w/o svc cuts on Blue & Orange), it would need about 23-24 trains

For a final comparison, If you use the red line, which takes about 148 mins to do a roundtrip, and has 10 min headways, you need about 14-15 trains. That number shoots to 18-19 trains to do 8 min headways

Take all this into account with trainsets being 6- or 8-car trains and you basically have how headways work and why losing so many train cars was detrimental

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