Union Pacific does not have a stockpile of parts for steam engines lie around there steaman find out a lot from watching the
youtube cahell and the refurbishment of the big boy 4014. Most of the parts on all of the steam engines have to be machined new or taken off of one of two spare parts engines they have. They have one that is a sister to the 844 and one that is a sister to 3985 both of which doin't have a lot of spare parts on anymore as most of them have been used for those two.
Also, the FRA rules regarding steam engines cover all seam engines operation in the US which is why many tourist railraids don't operate steam as often or will have more than one so that one down for its boiler inspection and any repair work they need to do to them. Many of them do not have the money that Disney does though to be able to schedule it as often can result in longer downtime for them. Also depending on how well maintained the engine is on a regular would depend on what and honeed to be relpalced, although most parts on steam enges were built to last and other than the boiler and stay bolts and wheels not very much acttly gets replaced. A few examples one of the ones at Disneyland engine number 3 was built in 1894 and it will be 127 years old in August of this year and may well have a number of the same parts that were used on it then.