The Related Cos. is a bit closer to paying rent on the West Side rail yards--valued at about $1 billion over the course of its 99-year lease with the M.T.A.

The commercial Architecture Billings Index has, for three months now, been slightly over 50 (it was 50.4 in July). Scores above 50 mean that billings increased month-over-month. Based on Related's deal with the M.T.A. for the development rights on the 26-acre site, Related must close on the deal and begin paying rent once three economic triggers are hit: the ABI must reach at least 50; the midtown office availability rate must be at or below 11 percent; and the average Manhattan co-op and condo sales price must pass $1,200 per square foot (this last trigger is slightly more complex).

Of course, all of these must be triggered at the same time, and, given the volatility of the economy, the ABI could fall below 50 once again.

As for the other indicators, there's a bit of breathing room for Related: CB Richard Ellis put the midtown availability rate for August at 13.5 percent (it's on the way down--it was 15.2 percent a year earlier), and Miller Samuel put the average Manhattan co-op and condos sales price for the second quarter of the year at $1,051 a foot.

ebrown@observer.com