A development during C. S. Lynde's regime as C.M.E. was the rebuilding, in 1928, of two Ba class locomotives with modern, wide firebox boilers. This resulted in a useful type of engine for freight service on branch lines.

The new boiler had a centre line pitched 7' 4" above rail level, compared with 6' 3" for the old boiler. The grate area was increased from 16.7 sq. ft. to 26.4 sq. ft., and the boiler pressure was raised from 175 lb. to 200-lb. sq. in. This increase in working pressure gave the rebuilt engines a tractive effort of 21,200 lb., as against 18,500 lb. originally.

At the time of their appearance in service the modernised Ba's were undoubtedly the most massive-looking locomotives in the South Island, although their total weight fell short of either the A or Ab class Pacifics.

Because of the increased height of the boiler it was necessary to fit squat boiler mountings, including the Great Central type of funnel favoured by C. S. Lynde. At first only one sand dome was fitted, but at a later stage, when a cab was provided on the tender for added protection from the weather in shunting service, a second sand dome was added.

The rebuilt Ba's three more were equipped with large boilers in 1948 and 1949 were always Otago engines. They were seen in service on various branch lines, including the Otago Central, and in later days used chiefly for shunting at Dunedin and for short-distance freight haulage in the Dunedin industrial area. Some were transferred to Greymouth.

Rated to haul 700-750 tons on the level, a wide-firebox Ba could surmount the notorious Mihiwaka Bank (north of Dunedin) with 270 tons 50 tons better than one of the other members of the class fitted with a narrow-firebox boiler.

In March 1928 No. 552 was rebuilt, this time with a high-pitched wide-firebox boiler. The modifications allowed it to haul heavier mainline trains. Much of this locomotive's work was on the Otago Central line, and it was derailed on one of the many tight curves soon after rebuilding. In November 1947, Ba552 was fitted with an all-weather cab, rear cowcatcher, and additional sand dome thus equipping the locomotive for shunting work, and it spent some time on the West Coast between 1957 and 1961. In 1964 it was given a full A grade overhaul, receiving another wide-firebox boiler. No.552 was used on railfan excursions between Dunedin and Waitati on two occasions, in December 1964 and March 1967. Through 1968 and until June 1969, it was retained for standby purposes, but was then withdrawn from service.

Mr L. J. Hostick pressed for the preservation of Ba552, and used it on another excursion, this time to Mosgiel in November 1969. In September 1970 it featured on a steam delivery trip between Palmerston North and Frankton, assisting A423. Ba552 thus became the only member of the class to work in the North Island, and for the greater part of the journey was the lead locomotive ahead of A423.

Ba552 is owned by Mr Hostick and was moved to Auckland in company with Bb144 and Ja1275. Ba552 has now been refurbished and returned to mainline service. It has been used on a number of excursions in the local Auckland area and featured at the Glenbrook Vintage Railway Steam Festival in 2002. It resides at Mainline's Auckland depot and can often be seen in use on Open Days.