Simplify the following text for a Grade 4 native English-speaking person.
Original: The first prototypes were built in 2006 and tested out of service from 2008 , the main fleet introduced into revenue service from 2009 to 2012 .
Simplified: The first train was built in 2006 . Testing started in 2008 , and the rest of the fleet will be delivered between 2009 and 2012 .