 To cope with the steep gradients of the Coimbra system, they were fitted with 600-volt motors, and each trolleybus carried two compressors, normally designed to work together, but each capable of maintaining the air supply for braking and door operation if one should fail.Three more Sunbeam Park Royals joined the fleet in 1956.Subsequent procurements included 10 BUT RETB/1s (four in 1958 and six in 1961), and six further Sunbeams in 1965.