I was flicking through heritage railway today and saw an article on the renovation of the NRM's replica rocket. the article was focusing on the fact that given the boiler, frames, and one other component which i have forgotten has been replaced, its verging on the realms of being a new engine. bearing away from the philosophical debate about grandpa's broom, why has so much been replaced on this engine when it is barely 20 years old? its not as if the engine was worked hard regularly, doing high speeds pulling long trains on the mainline? i'm not normally interested in NRM bashing, particularly as most of it relates to engines not being steamed and this one is, but it just seems a bit odd that so much of a relatively new, lightly used engine is being replaced. Does anyone know why?