Simplify the following text for a Grade 5 native English-speaking person.
Original: The first 2009 Stock train entered passenger service on 21 July 2009 and a new train will enter service every two weeks from early 2010 .
Simplified: The first 2009 Stock train started running on the line with passengers on board on 21 July 2009 . One new train will enter service every two weeks from early 2010 .