When the Shanklin line was electrified in ?1967, the floor of Ryde Tunnel was raised to avoid/reduce the regular flooding that had occurred previously. Flooding is not compatible with third rail electrification!!
This decision led to a reduced loading gauge suitable only for deep tube stock (or perhaps also D-stock). Presumably the electric D-stock could be introduced with minimum electrical modifications. However, there are rumours that they wish to remove the third rail electrification - to be improved with diesel power instead!!
The Isle of Wight loading gauge has never been generous ... but I suspect if the third rail were taken out and the Ryde tunnel lowered again, D trains could get through. De-electrification seems a retrograde step, but I thing that are precedents in Newcastle and Lancaster? And of course putting D trains on the Isle of Wight would be one of the few places they could be described as "cascading to more modern stock".
