I'm willing to write it off to too many narratives with too many different contexts.
The original BCS document laid out a service plan which was 15 minute on most lines but more on some.
There was a point when ML began talking to the P3 proponents where they became effusive about closer headways, mostly because the proponents were pitching that as doable.
And I'm sure the system designers always intended to build in a capacity well beyond 15 minute headways.So we saw plans for new layover yards in places based on some eventual view of the system.
And then as a matter of wishful thinking, spectators (including many Uters) came to see the closer headways as a given, when it might have only been aspirational or a 30-year design spec..
I expect that 15 minute headways will be the best we see on most lines, probably for a decade or more. There may be exceptions, but it will be a huge accomplishment just to get to that point.
- Paul