LONDON (Reuters) - Part of London s busy underground train system was closed on Friday following a bomb attack which police are treating as a terrorist incident. Trains between Wimbledon and Edgware Road in west London on the District line were suspended swiftly after the incident.  A further stretch of the District line between High St Kensington and Edgware Road which had reopened was closed again due to the attack which injured 22 people.  It is related to the Parsons Green incident,  a spokesman for Transport for London said. 