The homeowners above the tunnel should, I think, welcome the geotech study, as one of them apparently does, even if they aren't in love with an expensive bike path using it. The ends of the tunnel may be plugged, but the rest is still there, and given its age and 50 years of non-maintenance some of it must be a bit precarious by now. Was it concrete-lined or wood? A tunnel collapse could propagate upward causing building damage. Better to find out about it now and, if needed, reconstruct or fully plug the tunnel for stability purposes.