I checked the FasTracks site
, and I found:
The West Corridor line is an extension of the existing light-rail system. It branches off near the Auraria West station and then goes 12 miles west to the Jefferson County Government Center in Golden. It is currently under construction, and it should be done by 2013.
There are some light-rail extensions in planning:
A short southwest extension from Littleton
A short southeast extension from Lincoln
An extension along I-225 from Nine Mile to Aurora and the East Corridor line. Its Draft Environmental Evaluation has been released.
A central extension from 30th and Downing to the East Corridor line.
All the other lines are in existing intercity railroad track.
The Gold Line will run a few miles north from Denver Union Station then some miles west in the UP/BNSF tracks to Arvada and Wheat Ridge, for a total of 11 mi. It will be electric. Its Final Environmental Impact Statement has been released.
The East Corridor line goes 24 miles from DUS to Denver International Airport, in UP tracks for part of the way. It will be electric. Its Draft EIS has been released.
The North Metro Corridor line goes 18 miles northward from DUS in UP tracks. It will be diesel. Its Draft EIS is still being worked on, but it is almost ready for release.
The Northwest Corridor line goes 41 mi from DUS to Boulder and Longmont. It will be diesel.
FasTracks's planners decided to make some lines electric and some lines diesel as a result of their cost-effectiveness calculations. These included the current price of diesel fuel at when the calculations were made, but that has proven to be rather volatile over the last year. I suspect that they may make the North Metro line electric, but likely not the Northwest Line -- it's the longest.