Cal. Sts. & High. Code § 140

The department may establish and maintain shops for the construction, repair, and servicing of any equipment owned or used by the department. The department may purchase and supply such materials and parts, and furnish such labor, as is necessary in the construction, repair, and servicing of equipment for other state departments. The other state departments receiving them shall reimburse the department for the cost of such materials, parts, and labor, including overhead charges.