As another poster pointed out, there is no MSTS2. The company that originally developed MSTS and did some work towards MSTS2, but came up with their own sim, RailWorks, which is supposedly MSTS-compatible.
Kuju went from MSTS to RailSimulator. RailWorks is a repackaging of RailSimulator with a few (and I mean, very very few) bug fixes by the new owners of the code (A company that was formed to produce payware locos for RailSimulator called 'Rail Simulator Developments LTD' - aka RSDL).
Neither RailSimulator nor Railworks are 'MSTS Compatible' per se, although they do use some of the MSTS tools for content creation, and the keymappings are similar.
Kuju were working on the first MSTS2 (and pretty much completed it, by all accounts), before leaving MS to work with EA on 'RailSimulator', so in some aspects RailSimulator is likely very similar to what the first MSTS2 would have been like, although the Kuju MSTS2 was an expansion of the original MSTS 3d engine, rather than a rewrite.
The second MSTS2 is the one that many run into on the net, as the videos on YouTube and the MSTS2 website still exist. This had NO input from Kuju at all, and was developed by MS's 'Aces' department - who were also responsible for MS Flight Simulator. When MS axes Aces last year, any hope of MSTS2 finally died.
However... Aces Studio decided to go it alone, and may well be working on their own train simulator, they are now 'Cascade Game Foundry', and their press release when they created the new company also announced that their first product would be 'The light at the end of the tunnel - a new simulation project', which rather heavily hints at a train sim. That said, they also said 'keep posted for news in the next few weeks', this was over 6 months ago.