Hi Eno,
The cab is custom built using the same methods used in my 1963 VW Beetle Driveable asset, where the interior is just an invisible plane with the controls attached to it and the exterior makes up the cab through the use of the outside-visible-from-cabin and cabin-visible-from-outside tags.
It has fully working controls and the interior was built to IRW drawings from 1948 - 1951, and has been tested extensively. The bug where some controls aren't able to be clicked on in TS12 has been avoided, therefore the controls are fully useable.
Kieran.