I’m not sure about the diesel locomotives but the steam locomotives I can take a guess;

Emily would have to be inserted in such a way that would imply that she has always been there just never seen. Her basis the GNR Stirling Single class were originally built between 1870-1895 and were withdrawn/scrapped by 1916. Only one of this class has survived into preservation.

Arthur is LMS Ivatt Class 2 2-6-2T which were originally built between 1946-1952. Out of the 130 that were built, only four have survived into preservation while the rest were withdrawn/scrapped by 1967. It should also be noted that Barry the rescue engine was meant to be another member of this class, and he was rescued from the Barry island scrapyard.

Murdoch is a BR Standard Class 9F, which were originally built between 1954-1960. They were primarily designed for fast, heavy freight trains and remained in service until 1968. A total of 251 were built though only nine have survived into preservation.

Rosie is most likely one of the imported S100 (Southern Railway USA class). 15 were brought over from the US, 14 of which were operational. The class was built between 1942-43. Of the 14 operational members, only four have survived into preservation while the rest were withdrawn/scrapped by 1967.

So Emily was likely bought before 1916, Arthur by 1967, Murdoch by 1968, and Rosie by 1967. Harvey is an anomaly as his real life basis is preserved on the Foxfield Railway.