The man, who was not identified, walked onto the tracks in the median of North First Street near the Gish Station at about 9:15 p.m. and was struck by a southbound light-rail vehicle, VTA spokeswoman Brandi Childress said.
The victim did not appear to have used a crossing designated for pedestrians, Childress said.
"He was definitely in the right-of-way where he shouldn't have been," Childress said.
The man was removed from beneath the train and transported to Santa Clara Valley Medical Center, where was treated for serious injuries and is expected to survive, Childress said.
"He was awake this morning," Childress said.
The Santa Clara County sheriff's transit patrol is heading the investigation into the collision.
Investigators are looking at video surveillance images to "get a better understanding of where he was coming from," Childress said.