By tube (underground). Take the Northern line from Euston to Tottenham Court Road and change onto the Central line to Marble Arch.
Marble Arch tube station was created in 1900.
Euston tube station was created in 1968.
Tube station for Marble Arch is Oxford Circus
Marble Arch is a monument at the northeast corner of Hyde Park in London. The Marble Arch Tube station is at the west end of Oxford Street and at the south end of Edgware Road, across from the park.
Euston Railway Station in London, England, was built originally in 1837. As the United Kingdom's sixth busiest railway station, Euston Station was rebuilt and expanded from 1962 to 1968.
Marble Arch was originally completed in 1833 as an entrance to Buckingham Palace. In 1850, the Palace was remodelled and the Marble Arch was dismantled and moved to its present position the following year. The surrounding area is now known as Marble Arch with the nearest underground (tube) station named after 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.
The best option is to walk to Euston Square Station - which is well signposted and a two minute walk from Euston Station. Then, take the Hammersmith and City Line, Circle Line or Metropolitan Line to Liverpool Street.
Yes, all London mainline stations have an underground station as well.
Berkhamstead is too far out to be served by the tube. There is a mainline service to London Euston.
4.7 miles by road.
Hyde Park Corner, Green Park and Marble Arch are all about the same distance from Mount Street.