According to the Mercury-News:
"What particularly appeals to Caltrain about the high-speed project is the proposed widening of its tracks and construction of grade separations up and down the Peninsula because bullet trains must run above or below street level."
"Caltrain wants to replace its aging diesel locomotives with lightweight electrified cars, which will require a $785 million overhaul of its infrastructure by 2015. The agency hopes that the planned path for the bullet train — up its right of way between San Francisco and San Jose — means the two rail systems will be able to share the cost of upgrading the tracks. Officials hope all of Caltrain's advance planning work will make the Peninsula a strong candidate to become the first part of the line to be built, said spokeswoman Christine Dunn."