The rail line is scheduled to be open in 2016. Here's more info from RTD:
The rail cars were pulled along the new tracks from Union Station along the 23 mile-long route at a pace of roughly 10 miles per hour.
This kicks off a year-long schedule of testing and commissioning prior to opening day in 2016.
It was the first time people were able to see the commuter trains on the tracks to the airport. A diesel-powered "track-mobile" pulled the cars. The overhead electric wires were not used to power the trains on this trip.