The 901, which was benched in WIL for over a year as Acelaphillies mentioned is the dedicated unit for the ACS-64 test trains. 905 hasn't been involved with the test train to date. If you recall, the 901 was traveling with the test train because it would turn at various interlockings to perform tests. Without another unit, you'd have to cut the ACS off and run around the train in the middle of nowhere.
Once they completed enough tests and made certain adjustments (which clearly didn't help yesterday, right Approach Medium LOL!!!!!) they started the cab car tests. There are a few hiccups here and there, but nothing too major I suppose.
Speaking of tests, the 603 sandwich is on 284(20) and 68(20). At NYP, they'll yogi it to a lite engine move to WIL at some point Sat.
I want my road foreman!