The Caltrain Modernization Program (CalMod) is a $1.9 billion project that will add a positive train control (PTC) system and electrify the main line of the U.S. commuter railroad Caltrain, which serves cities in the San Francisco Peninsula and Silicon Valley, as well as transition from its current diesel-electric locomotive powered trains to electric multiple units (EMU). According to Caltrain, electrification of the tracks will allow it to improve service times via faster acceleration and shorter headways, reduce air pollution and noise, and facilitate a future railway tunnel into downtown San Francisco's Transbay Transit Center (as diesel trains cannot serve underground stations).