I don't think that splitting TS 2013 into different parts is practically possible.
For example - How would you alter the railworks.exe file so that it opened the correct continent/area. I guess that you could have multiple railworks installations with separate .bat files used to open each discrete part of the game but IMHO it would be a nightmare to keep up to date, and ensure that you had all the correct Assets and Content etc for each area.
Steam (Steam tool Library Manager) already gives a basic option of installing your games on separate drives or separate partitions on the same drive (as gptech intimated on UKtrainsim) You can also use the mklink function or via "STEAMMOVER" to do that if needed. But that only covers complete games on separate HDDs.
Like here
http://forums.steamgames.com/forums/sho ... ?t=3103869 and
http://www.traynier.com/software/steammoverIt may be worth posting on trainsim.com to see if Otto has any ideas on how to that as he seems to have multiple installs?
http://www.trainsim.com/vbts/forumdispl ... RW-GeneralI guess that with SSD prices falling (in some cases <$1/GB) then soon large SSDs will become commonplace, and you can also get some pretty fast conventional HDDs these days that would make loading, etc quite fast.
Regards
PeterH