Toward the end of the delivery period of the original C5 4-6-0s, their boiler capacity was already barely adequate for keeping time with the ever-heavier trains of the belle époque. Belčamin had already started work on a new boiler for the C5s a year earlier, and was able to fit it to the last ten new engines to be delivered. By 1901, a further 20 locomotives had been rebuilt, becoming class C5A. The larger grate and higher pressure made a big difference in performance and economy. The crews were not so keen, however. The engines were considered difficult to fire, which was blamed for a frequent shyness of steam, whereas the older C5 class was trusted and revered. The upshot was that Belčamin had to design another boiler, with which over half the class was rebuilt from 1901 onwards, becoming class C5B.