Cab view of A line from DUS to DIA:
Train to the Plane arrives April 22
The line is almost all double-tracked except for a few single-tracked parts.
The train departed from the easternmost track of DUS and went north, then east. That eastward part paralleled some Union Pacific track, and there was a thick barrier in between. Protection from derailment? Much of that track was slab track. Is that for flood resistance? That's about the only reason I can think of for at-grade slab track away from road crossings.
Near the Peoria station, about 2/3 of the way into the video, one can see the I-225 light-rail line's tracks on the right. That track turned away a little after the Peoria station -- southward a little east of that station.
A couple minutes later, there was a third track to the south of the main two, and a bit after that, the two tracks merged to make a single track. However, there was room for a second track to the north. Then a viaduct over the UP line, which was single-track there, and over some multilane flat roads. When it ended, the track doubled and went into a station. After continuing, the video did an abrupt jump to DIA station.
I consulted Google Maps for what was next. Another station, then the tracks merge to make a single track, though with room for a second track. This included some bridges, all wide enough for a second track. When the track turned northeast, it doubled again, and remained doubled almost all the way to the airport. However, the track construction ended a little outward from the X-box crossing, so Google Maps was not quite up-to-date. Bing Maps was less-up-to-date, as was MapQuest. Curiously, different resolutions of MapQuest's satellite imagery looked like they came from different mapping runs.