The liability question is most interesting. British Railways Board had a considerable number of closed properties on its books until 2013, including Chelfham Viaduct on the L and B, a line which closed some 13 years before BR even existed - British Railways inherited the Southern’s liability.
The SLNC liquidators managed to sell off the NI assets within a year or so of closure, but struggled for quite a while to sell Weirs Bridge as it would have cost any contractor so much to dismantle.