From the changelog

Added train part buying

• To buy a missing part, you need to reach step 3 (Order Missing Parts) of the vehicle restoration process
• Paying is done at the museum’s office cash register
• Once paid, the missing part will await at the designated loading track
• To pick it up you must bring a utility vehicle to the loading track (jump to 6:58)
• Interact with the cargo transfer machine, like you normally would for freight
• Bring the loaded utility vehicle to the museum and park
• The part will be automatically unloaded and installed to the vehicle it’s intended for
• Transported train parts cannot be damaged