Buenos Aires Great Southern classes 6, 6A and 6B

In 1888 Beyer, Peacock & Co. built eight 4-4-0 locomotives for the Buenos Aires Great Southern. Two of these were built as Worsdell-von Borries cross-compounds to compare them with the six simple locomotives. The simple ones became class 6 and the compound ones became class 6A. In service it turned out that maintenance of the compound variant was not more complicated compared to the simple variant, while consumption of water and coal was less. In the result of the comparison, Robert Gould wrote that simple engines were only better in shunting service and local traffic. This was especially true for railways without many steep inclines like this one.

This meant that further locomotives were also to be built as compounds. These were designated class 6B and 22 were completed in 1890, also by Beyer Peacock. These featured a larger boiler and cylinder diameters which had been increased by one inch. Before production started, the design of the automatic valve had to be altered. The original design tended to produce sluggish jerks when it switched from simple to compound mode too early.