Whilst I have not seen the history (or is it historic) aspects of this thread and with some background in how these things work, multi-ownership is rarely the issue.
It is more likely the case that the underpass will belong either to the operational railway estate or it will belong to the local authority.
Less rarely (but not common), disputes do arise about who that owner actually is and correspondingly who has the maintenance and upkeep obligations.
If this type of dispute is in play - shame on the respective parties as they probably have had ample time to work out a resolution, (it has been 18 years since British Rail was privatised in April 1994).
On a contributory note as to what actually may be done, there is a lot of of local history that could be depicted in a mural or montage - the original canal, the pear orchards, the links to Ashberry Cottage, 18th C home of William Duke of Clarence, later King William IV and his mistress Dorothea Jordan and other historically famous individuals and so on.
There are plenty of people who contribute here who could more positively argue the merits of inclusion of these type of depictions than me.