The woman, who was visiting from Indiana, was declared dead at the scene.
"I was looking out the window, and I just see a mangled car, the right side of the car was completely destroyed," Burlingame resident Lidia Damico, 26, said by cellphone while waiting on the train that struck the car. "And I take it the driver of the car or maybe a passenger was running around it, looking frantic. And I just saw pieces of debris around the car."
Friday's tragedy unfolded at about 5 p.m., during the busy commute hour at Charleston Road and Alma Street. It was then that the northbound train No. 369 speeding up to 79 mph plowed into the car, transit police said.
Officials said the couple, who were in their 60s but were not immediately identified, were visiting family in Palo Alto. Her husband, the only other passenger in the car, fled the vehicle before the blow, and it's not clear if the woman ever made an attempt to escape. But authorities said Friday that the car had stopped on the tracks due to backed-up traffic.
Witnesses describe a frenzied scene as emergency vehicles descended on the car.
Sethu Anand, another passenger on the train, saw a glimpse of the car just after the train sped past the intersection. "The back part of the car was really, really damaged," Anand said. "The (crossing) signal was down. A few people (on the train) realized that something was hit, but others didn't even notice it. It was really fast."
It was the sixth death on the tracks this year. Most fatalities on the tracks involve pedestrians, although vehicles sometimes get stuck on a crossing.
On New Year's Eve 2010, a Caltrain struck a blue pickup at Auzerais Avenue in San Jose and killed its 43-year-old driver. In August 2009, a Caltrain struck and killed a man sitting inside a Honda at a Redwood City crossing.
Safety experts advise motorists to get out of their car and run away if their vehicle gets stuck on the tracks while the crossing arms come down, since it is only a matter of seconds before a train arrives -- and the locomotive can't stop in time.
"It's easy to say, 'Well, why didn't they get out of the car?' ... until you're sitting in the car with the train" heading toward you, Caltrain spokesman Mark Simon said. "It's hard to know how you'd react."
Bay Area News Group staff writer Diana Samuels contributed to this story. Contact Mike Rosenberg at 408-920-5705.