The shooting took place at the Santa Clara Valley Transportation Authority (VTA) rail yard in San Jose.
Officials say the victims include multiple transit employees. The suspect, an employee, is also dead.
The shooting broke out around 06:45 local time (14:45 GMT).
Officials say the suspect may have set his home on fire before the attack.
Across the US, there have been 230 mass shootings so far this year, according to the Gun Violence Archive. A mass shooting is defined by the group as a crime in which four or more people are fatally shot.
