Mostly used for switching around the yard and maintenance of way as you say. As rebuilt, it now has an air brake compressor and controls, which it never hd before. Part of the diesel driver's test for the railway (which I passed!) involves controlling a train down the 1.5% grade using engine braking only, as the hand brake was good for parking only! It has a 3 speed epicyclic gear box.
The locomotive was one of many hundreds built over many years in various sizes, but to the same basic design, See
Ruston Locomotives
Don Newing
Derbyshire, UK