California Code, Revenue and Taxation Code - RTC § 6411

(a) The storage, use, or other consumption in this state of tangible personal property becoming a component part of any railroad equipment in the course of repairing, cleaning, altering, or improving that railroad equipment outside of this state, and charges made for labor and services rendered with respect to that repairing, cleaning, altering, or improving, are exempt from the use tax.

(b) For purposes of this section, “railroad equipment” includes locomotives, freight and passenger cars, maintenance of way equipment, and any other equipment riding on flanged wheels and owned or used by a common carrier engaged in interstate or foreign commerce, or by any person for the purpose of leasing that equipment to a common carrier engaged in interstate or foreign commerce.