The diesel locomotives later called class 24 were designed as a part of the 1955 modernisation program and initially carried numbers starting with D5000. Their second name “Sulzer Type 2” gives reference to the origin of the diesel engine and the power class. One requirement was that as many parts as possible could be taken off the shelf from the industry.

The engine of type 6LDA28 was a direct-injected and turbocharged six-cylinder that was the same as in the CIÉ class 101, but with an increased output of 1,160 hp. Other parts like the generator, traction motors, brakes, multiple working equipment and steam heating boiler were also taken from existing types. On the cab ends they had gangway doors, but as in other locomotives they were rarely used and later got sealed.

Some belonged to the lighter subclass 24/1. These had smaller fuel and water tanks to increase their route availability in the Scottish highlands. Most class 24/0 and 24/1 locomotives later lost their steam heating boiler when they were no longer used in passenger service. They were withdrawn between 1973 and 1980. Today, four are being preserved with all of them again carrying their pre-TOPS numbers.