Perhaps, I should've been more specific... Yes, rolling stock and scenery items, but not routes or sessions.
WEN thinks there must be some way to decompile the cdp files to gain access to the original mesh - else how did his class 121 and class 141 manage to get ported over. WEN's class 302 and 308 have also been ported over (legitimately, using supplied meshes) to Railworks by a former TRS user and appeared - after some work - to work well in it, so it is possible. I think WEN's class 502 is being optimised for both Railworks and TRS, so again it is possible.
More over, they would have to of had a copy of Trainz to do it (and the "creator" denies having a version of trains, hello, 141 only available on the DLS and so does the 121.). The textures are straight-forward to do as you can easily convert them from BMP/TGA/JPG/GIF/Whaterver to ACE (Railworks format) easily. The problem is decompiling the im/pm files, opening them up in another program and then exporting them as a IGS file. That itself is possible, but I only know of one program that does it as I've had to use it for my own meshes.
The problem isn't conversion, to make it clear, you don't own the object after downloading it, you accept to use a licence, which allows you to use the content, if you don't like the licence, don't download it and make your own. If the licence says you can't use the object on Wednesdays for the next 1,345 years, then you have to abide by it. (The interesting point, would that be legally binding, let alone enforceable?) That was just a small example, although rather extreme.
However, the EULA as its called is one of the first things to be ignored - one reason why I don't have EULA's, rather than rely on common law for virtually everything. So you can't take me to court because you created it before I did, as I have plenty of evidence to say I made it

. Most creators say "you aren't allowed to do x, y, z without permission" as that is the restrictions they have in force and they may have a number of reasons for having those restrictions (Think about it like this, if your work that you slaved away for 3 months and then someone claims it as theirs on another site and distributes it, wouldn't you be upset? I am sure you would be. Its called reasonable respect.) And that's what ultimately the authors would like, respect.
That is why it always pays to ask and email "would it be ok if I do this to your object?" to the author via email? No one is going to have a go at you, in fact, you might get a yes in a majority of cases or a long explanation to why you cant. And if an author doesn't get back to you, its still not OK to just go ahead. Quite simply, if you just go ahead doing what you want to without the authors permission, then oh dear, if s/he recongises it, you could be in a lot of trouble and that may mean civil court action.