I am no expert on guage width, viaducts or tunnels. All I would add is that I live in FH and work on the Strand. Currently I go by train from FH to London Bridge and then squeeze into any train going to Charing Cross. Annoyingly there are plenty of trains from Charing Cross to Catford Bridge / Lower Sydenham, so any link between Catford and FH would be welcomed by me, in whatever guise. I think there must be lots of other people like me who are frustrated that there are (or were) trains going from FH to Charing X in off-peak hours, but not in the rush hour. The change at LB is the low point of my commute.