Euston. Network Rail's headquarters are located outside Euston station.
Alternatively, you might be asking which is the nearest tube station to the National Rail in London. In which case, there are loads of railway terminals in London and each one has its own tube station.
The nearest tube station is Canada Water station.
Romford doesn't have a tube station but the mainline station has a frequent service to London Liverpool Street. The nearest tube station is Upminster.
Camden Tube Station
Nearest to what
If you mean the Royal Hospital, Chelsea - which isn't a hospital, it's almshouses for retired army personnel - the nearest main line station is Victoria and the nearest underground station is Sloane Square.
Arsenal
Westminster
69
Leicester Square has its own tube station.
Westminster or Waterloo
The nearest tube station is Waterloo.
Oxford Circus