In case you didn't already know... you can follow the new cars as they debut today with my app...
https://sites.harvard.edu/~wuensch/T/subway-map.html
The trick is to hit the button "Show a detailed table" under the map, and then click the header on the column "Lead Car No." to sort them in order. Look for anything with 14XX and that's the new Orange Line cars.
Also of note - 3900 & 3904 are signed for Gov't Center and inbound at Reservoir right now. I bet they are planning on making some show of both new types at North Station - or something like that.