Today was the first day of revenue testing on the R211s - they have to run trouble free in customer service for 30 days before transit fully accepts delivery on them - as of right now they still technically belong to the manufacturer. Any sort of glitch restarts the clock. The R179s which are the other "new" trains running on the A and C (and J) lines had lots of teething problems so you want the bugs worked out before you take delivery on a bunch of trains.

To put it in perspective, these R211 cars have been on property for 2 years doing various tests with nobody on the trains, training crews on the new equipment, etc before getting to this point.

The 211Ts which are the open gangway cars only came on property fairly recently - so they too have to do the empty testing first.