There are 5 tube stations within walking distance of Trafalgar Square.
Charing Cross, being the main one, approximately 4 minutes walking at an average speed.
Leicester Square, a an eight minute walk.
Piccadilly Circus, also an eight minute walk.
Covent Garden, is an eleven minute walk.
Embankment, is also an eleven minute walk.
The Tube Lines that serve these stations are…
Charing Cross; Bakerloo and Northern.
Leicester Square; Piccadilly and Northern.
Piccadilly Circus; Bakerloo and Piccadilly.
Covent Garden; Piccadilly Line.
Embankment; Circle and District lines, as well as Bakerloo and Northern Lines.
Jubilee Line can be found at Westminster Stn.
Victoria Line can be found at Victoria Stn.,
Waterloo & City Line can be found at Waterllo Stn.,