
An important factor to mention in the case of the HSTs and 91s is that these trains cost very little in accounting terms, as the HSTs are probably fully depreciated, and the 91s a large extent depreciated. If infact these sums are counted on ROSCO balance sheets as the ROSCOs didn't pay for these trains. Some research done recently on the lifetimes of steam locomotives showed that BR in the 1950s had little incentive to replace surviving 19th century steam locos as they had fully depreciated long ago. No doubt the continuing survival of the Class 20 is partly related to this.
Aditmittedly, normal economics are not known to work well in the rail sector. I must read up on how Virgin financed their new trains...I suspect that could be an interesting case study.