Federal investigators reported Sunday that the driver, who has not been named, had turned off the train's automatic controls moments before the collision.
Lum said drivers under pressure to keep their trains running on time turn off the controls before entering the West Portal station to speed loading and unloading.
Four passengers were seriously injured in Saturday's crash, including the driver.
The light rail is part of a mass-transit system that carries more than 200 million passengers a year in San Francisco, including on the city's famous cable cars and historic trolleys.
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Associated Press Writers Evelyn Nieves and Tim Reiterman contributed to this report.
