The A-Train is a train that runs from Denton to Carrollton and includes an interface with DART's Green Line. It started running on June 18, 2011, starting with a week of free rides.
The most exciting thing about the A-Train for party people is that it is a means by which to reach Dallas. Unfortunately it doesn't run late enough to go and get drunk, usually. At least, not if you plan on returning home before morning.