Simplify the following text for a Grade 11 native English-speaking person.
Original: There are 66 trainsets which is made up of C151 , built in 1986-89 by Kawasaki Heavy Industries and collaborating with three sub-companies , Kinki Sharyo , Nippon Sharyo and Tokyu Car Corp. .
Simplified: The majority of the fleet comprises 66 six car C151 trains ; these were the oldest trains in operation . They were built between 1986 and 1989 by Kawasaki Heavy Industries in consortium with Nippon Sharyo , Tokyu Car Corporation and Kinki Sharyo for S$ 581.5 million .