Ofificially...?
It is mainline US Airways, the EMB-170 Division of mainline US Airways.
Reality...?
The thing is that MDA is a strange hybrid mix of mainline and express, you basically just have to be flexible and ready for some changes from what the norm was back at mainline
For example:
It is Midatlantic, with MDA as the usual initials, as MD is the Airline code for non rev use, but the flights are "USAir flight so and so", and the Fed's are very particular that it is reffered to as a division of Airways, not separate.
It is a little strange, as the ID's are Airways ID's, not express, but they have a strange blue MDA logo under "US Airways". Your hardware (wings, emblem) are US Airways, not Express. Yet the Aircraft reads US Airways Express, not "US Airways"
The best way to think of Midatlantic, is simply that is is mainline's express division, flown by Airways pilots, but with express pay and express work rules (with the exception of the Sched. section of the Airways contract) Your Pay will be American Eagle TOS (8yr) if you are an F/O coming in off the APL 36ish), or 1st year Eagle CA pay (54 ish) if you are comng in as a CA.
You are back as a voting member of the Airways ALPA (AAA), and have to pay dues and assesments. But the assesments are scaled back to reflect the lower pay
Right now the classes average 10-12 pilots a week, one a week, each week. Sim training is up and running in both YUL and CLT now, and soon the ground training will transfer from the PIT school. The majority of the the class positions have been for F/O's to make up for the shortage here, but the mix should change into the future. There is not hard and fast number of CA v FO in each class as I have seen it shift a few times already.
The company has seemed to have been slow to post class awards due to the shifting changes in CA v FO numbers, (IMO due to dealing with attrition), this has been from btoh above with the APL pilots and below from the CEL (Wholly Owned) pilots. Wish I could tell ya something more useful than that for now, but like I said, things tend to shift and change here on a constant basis, so even if I did tell you one thing it is "subject to change"
Hopefully they will give you a decent idea in a short while. For now, the two bases are PIT and PHL, with a projected base in DCS opening this fall.
Hope it helps, GL