Open Rails is a bunch of guys who do not want to see the demise of MSTS. For many MSTS users, the size of their collections of rolling stock and routes is the only thing holding them back from upgrading to newer simulators.
As MSTS has been around for almost 10 years now, no other simulator is going to have the same level of content available for it, for years to come. However, because no newer simulators can/want to provide backwards compatibility for the ex-microsoft title, the content for it is doomed. Therefore a bunch of enthusiasts decided to try and create a newer graphics engine of their own based on Microsoft XNA I think, that would be able to read in MSTS content, but not require MSTS itself. Therefore prolonging the life of MSTS content. Open Rails is also intended to feature new functions and techniques that MSTS never had.
Open Rails was stopped from being available for download, because in its early development stage, too many users were complaining that it wasn't as good as MSTS. Clearly true, because those behind it have only just started out. Once its at an end user applicable stage I am sure they will make it available for download again.
Ultimately though, it will never take on the roll of an MSTS replacement, as it is being run by (for lack of a better term) 'bedroom developers'. The only people who will know about it are forum goers and hardcore fans who desperately want to hold onto MSTS. It will serve a clear purpose and be enjoyed by many though, I am sure.