You can easily reach Horse Guards Parade by train. The nearest station to Horse Guards Parade is London Charing Cross, which is served by Southeastern services throughout the day. Turn left onto Strand, then at the roundabout, take the 4th exit onto Whitehall until you see Horse Guards Parade on your right.
The iconic Horse Guards Parade is a parade ground and the scene of Trooping the Colour on the Queen's official birthday in June. You can visit the Household Cavalry Museum which celebrates the history of The Household Cavalry, you can even dress like a cavalryman!
Visit the Horse Guards Parade website for opening information.