You probably do not want to hear this, but I do not think there is a patch for the Steam version (unless Steam have actually released a patch, I will be very interested then)
Anyway, Steam installs it into a special folder, not a standard one.
Shane is correct - there is no patch available for that version (unless "Steam" have very recently released one, which I doubt, but you could try asking them...?!?!?).
Posted by Zec on the 19th January 2011 (copied & pasted from his post)...
It depends on where you purchase TS2010 from. The following is a list of builds (and if there are any further patches).
Digital Download (DGDL) from Auran store: 43434 (needs hotfix)
DVD from Auran Store: 42203 (requires SP3 + hotfix)
DVD&DGDL from JustTrains: 41491 (requires SP2 from JustTrains, then SP3+hotfix)
DVD&DGDL from US publisher (sold via gamersgate, amazon, and a few others): 43361 (no patches at this time, this build is SP3)
DGDL from Steam: 43361 (no patches at this time, patches would be released via Steam - note the patch system for this is separate to the US publisher's version if I remember right - this build is SP3)
Other regional versions also have a few other builds, and SP's are either on our website ( Trainz SP's ) or on the publisher's website.
That should, however give you an idea of which versions require what patch. The first one is a fairly quick and simple patch, being just a hotfix rather than a full service pack.
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Zec Murphy