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