Simplify the following text for a Grade 12 native English-speaking person.
Original: This makes it very versatile , although the diesel engine produces less power than is available from the third-rail supply so the locomotives rarely stray from the former Southern Region of British Rail .
Simplified: This makes them very versatile , although the diesel engine produces less power than is available from the third-rail supply so they rarely stray from the former Southern Region of British Rail .