The AB class compared more favourably in service against the compound A class. Reputedly the first engine able to generate one horsepower for every 100 pounds of weight (16.4 W/kg), the AB class was efficient and versatile, and the engines were easy to maintain and operate. However, it became clear that there were several shortcomings of the design – notably the cab was too small, and the tenders were not sturdy enough. A new and longer cab was fitted to all new locomotives being built from that time onwards, with the first appearing on AB 663. New tender structures were also built at a later date.