That sounds like an interesting approach to handle the common problem with encyclopaedic vehicle sets where at any one time you have 20-30 different models available, but usually one clear winner for each of passenger, express passenger, express freight and heavy freight which gets used almost exclusively.
The downside is it becomes a bit of a pain to clone vehicles on existing routes, but then that's nothing which doesn't exist in the real world. (A good example being South West Trains cascading Southern's old 455 and 456 stock to extend their suburban fleet).
I think a good gameplay vs. realism compromise would be to mostly follow the real life production dates, but extend them in places so players always have a decent locomotive to choose from in each of the high speed, high power, high TE or low running cost categories. And maybe some of the things which ended up just about everywhere (like the 4-CIG/VEP/etc. family) might stick around a few years longer than their real-life production run to simulate the sheer numbers that were originally produced.
It'd certainly make a change from some of the games I've played where I ended up using nothing but Moguls for a 40-year period because they're too good a compromise and having such a homogeneous fleet makes replacement and management much easier.
Thanks, it was the only way i could think of to eventually include so many different trains without having a massively busy purchase list.
For the most part you should never be short of a decent train, but it will encourage buying trains in batches and having a large variety of trains over time. (You can of course always enable vehicles never expire for the full list)