Take one of the frequent fast trains to London Paddington then hop on the southbound Bakerloo line to Oxford Circus. From there is it is simple cross-platform change onto the southbound Victoria line which gets you to Pimlico. Did this route every day there and back for a year :)
Pimlico tube station was created in 1972.
Pimlico
bus it
The Trainline web-site says - take a tube train from Euston to Paddington, then a first Great Western service to Reading. Journey time Euston to Paddington is 28 minutes and Paddington to Reading is 25 minutes.
You can find collections of John Constable's oil paintings of clouds at: The Victoria and Albert Museum (South Kensington tube station) The Tate Britain Museum of Art (Pimlico tube station) Both of these museums have free admission (although donations are requested), and each has 2-3 of Constable's cloud paintings in particular.
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.
Camden Tube 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.
Ruislip tube station was created in 1904.
Brixton tube station was created in 1971.
Stanmore tube station was created in 1932.
Borough tube station was created in 1890.