TRINITY RAILWAY EXPRESS (TRE)
https://www.dart.org/riding/tre.asp
Travel on the TRE between Union Station and CentrePort/DFW Airport Station or between Fort Worth T&P Station and CentrePort/DFW Airport Station
Or, if you don’t want the TRE, the Orange line will take you from downtown to Terminal A, where you can check in for any AA flight. (If you want to take another airline and check luggage, you’ll have to transfer to the circulator bus to the D or E terminals).
Of course, the 2 hour trip from Plano to DFW at rush hour doesn’t make much since, but I’ve seen folks doing it for the $2.50 fare.