Simplify the following text for a Grade 12 native English-speaking person.
Original: The British Rail TC ( Trailer Control ) multiple units were un-powered fixed formations of 3 or 4 carriages with a driving position at each end of the set , converted by BR at York Works from locomotive-hauled Mark 1 carriages in 1966-1967 and 1974 .
Simplified: The British Rail 4-TC multiple units were unpowered fixed formation 4 carriage set ( fitted with driving cabs at each end ) converted by BR at York Works from locomotive-hauled Mark 1 carriages in 1966-1967 and 1974 .