And then there's SF's Central Subway, a 1.7 mile underground light rail extension to Chinatown. I'm not sure when to consider a project under construction--they're underground along the new route moving utilities and whatnot, but tunnel boring has not yet begun as far as I know. Wikipedia lists the Central Subway as under construction:
Wikipedia:
The Central Subway is an extension of the Muni Metro light rail system in San Francisco, California, from the Caltrain commuter rail depot at 4th and King streets to Chinatown. The subway is the second phase of the Third Street Light Rail Project. Ground was broken for the new route on February 9, 2010, and is currently scheduled for completion by 2019.