Cal. Pub. Util. Code § 7606
Every steam locomotive used upon a railroad in this State, carrying passengers or freight for hire, shall be equipped with one or more water glasses of the type known as the "solid water glass," the same being a solid piece of glass with open flutings at the back thereof, which flutings will permit the raising and falling of water in the boiler of the locomotive to be plainly visible from each side of the cab without the use of a reflector. The glass shall not be less than seven inches in length and one and one-quarter inches in width, and five-eighths of an inch in thickness.
Any person, firm, or corporation operating any such steam locomotive which is not equipped with one or more water glasses as described in this section is guilty of a misdemeanor.
Ca. Pub. Util. Code § 7606