It's not quite connecting the transcontinental railroad, but for commuters who have sat in traffic for years at the Caldecott Tunnel, it's definitely a milestone.

The San Jose Mercury News is reporting that excavation gear in the fourth bore construction has hit the other side:

Caltrans contractors broke through the Orinda hillside on Tuesday afternoon, connecting the two sections of the tunnel being dug to create the fourth bore of the Caldecott Tunnel.

A giant excavating machine called a roadheader pierced through a wall about 2:45 p.m. deep inside the tunnel, Caltrans spokeswoman Ivy Morrision said.

The fourth bore is projected to open in late 2013.