This really doesn't have an easy answer, as there are multiple different 'types' of locomotive, and other variables.
The main one is, does the asset take advantage of any new functionality in the newer version? If so, then back-dating the model is likely to see issues, varying from 'fatal' (will not work) to 'minor'.
A few examples here. If the asset uses the newer 'tbumpenv' (or a few other) materials, then it will not work in versions prior to TS2009 SP1 (build 3.0).
If the asset uses the newer smoke effects for steam locomotives, then these would need to be entirely replaced to work in versions prior to TC3.
If the asset uses the newer steam locomotive sounds and physics, then they need to be replaced, and may require mesh changes to make them work correctly, in versions prior to TC3.
If the asset uses the newer bogey tags for sideplay and rotation, then it will need them removed to work in versions prior to TC3 (this will result in the bogies not 'following' the track, or loco frames, correctly).
There are many other functions that are also going to effect this, such as script functions. As such, no overall answer can be given.
Regards