For those having trouble installing due to failure to detect an installation, it's handy to know how these installers detect an installation. Basically, the installer looks into the Open Rails settings (in the registry) and looks for any installation profile with the name "TrainSimulations". The path corresponding to this profile is what each TS installer considers as the OpenRails install. (Even if it's not actually an installation of anything!)
Trouble is, if you were to say move the Mullan install to a different installation profile or rename the installation profile from "TrainSimulations" to "Train Simulations" or maybe just "TS" this completely breaks any new installer's ability to find the installation.
For example, the installer can't find my install as I've named my TrainSimulations profile "TS". I actually did this on purpose, as I really hate the installer not giving me the choice of where to install things. Issue is, that strategy backfires for this pack.

However, the solution is surprisingly simple. If I were to create a new install profile named "TrainSimulations" the installer works just fine.

Note how the folder name does not matter, only the name of the installation profile. Also note that you don't actually need Mullan installed in the same folder as the expansion (my "lol not actually trainsimulations" folder is completely empty, there is no Mullan or anything in there), which allows for separating new installs from old ones.
For example, it seems that the expansion includes some tweaks to content from the original Mullan. This isn't obvious unless you go out of your way to install the expansion pack to a folder which doesn't have Mullan at all.
Personally, I really dislike this. I belive the installer should always give the user full control over where it installs to, and using the registry to detect an Open Rails install should only be a bonus feature to make the installer easier to use. There should be no requirement to install to a specific folder, nor anything else to take control away from the user.
But does it have field shunts?