I'm surprised no-one has considered London Broad Street. The decline was long and slow, but given 27 million passengers in 1902 and one train per 2 minutes in the peak, travelling from pretty much everywhere (services running from the Midland, the Great Northern and the NLR), down to not existing as a station in 1986, I think it's a prime candidate for important railway centre that has fallen from grace. A slight reprieve over the Kingsland Viaduct from London Overground, but it's no Broad Street station (and certainly not 30 tph). It also had a major adjacent goods yard, which also no longer exists.
The Farringdon (Street)/Holborn Viaduct/Ludgate Hill area is also considerably reduced from what it now was, despite the Thameslink services and I'm not just referring to the removal of the City Widened Lines service to Moorgate. London Reconnections has a good blog post here:
http://www.londonreconnections.com/2012/london-terminals-fullsome-farringdon-part-1/
The area had a major freight section as well as having a City of London passenger terminus.
The Farringdon (Street)/Holborn Viaduct/Ludgate Hill area is also considerably reduced from what it now was, despite the Thameslink services and I'm not just referring to the removal of the City Widened Lines service to Moorgate. London Reconnections has a good blog post here:
http://www.londonreconnections.com/2012/london-terminals-fullsome-farringdon-part-1/
The area had a major freight section as well as having a City of London passenger terminus.