Simplify the following text for a Grade 13 native English-speaking person.
Original: They were fitted with the Sulzer 12LDA28C twin-bank twelve-cylinder unit producing 2750 b - though this was later derated to 2580 b to improve reliability - and have been used on both passenger and freight trains on Britain 's railways for over 40 years .
Simplified: They were fitted with the Sulzer 12LDA28C twin-bank twelve-cylinder unit producing 2750 b ( later derated to 2580bhp to improve reliability ) , and have been used on both passenger and freight trains on Britain 's railways for over 40 years .