For those who didn't get the hints in the contractor story, or don't read them, at all
Tesla moves to another station after contracts #2, #7, and #12
This means, that not only the theme (station background) changes, but also the buildings and decorations are gone because they do not fit in a train, so he can't take them to the new location
If for whatever reason you want to keep the buildings and/or decorations, put them in storage before you finish the contract(s)
FYI: Selling the buildings and/or decorations to earn gold is pointless, as there are no items in his shop that require gold