Although the heavy goods engines of the RSR were the first Doherty types to enter regular service, the cylinder arrangement was first tried out almost a decade earlier. It seems strange, looking back, that the "trial balloon" shown above was built at all. The purpose of the Doherty layout was to maximise tractive effort and a mere three coupled axles would hardly seem adequate to transmit the potential power. In fact, the 364A was in many ways a guinea pig, being used to determine optimum boiler and firebox dimensions for the new range of locomotives that Ketterik was already developing, to examine the advantages of steam-jacketed cylinders (as far as we know, only André Chapelon had used these on a locomotive before, in 1948), and to test improvements in steam flow through the use of multiple, smaller cylinders. This last objective explains why the engine received piston valves to begin with. Early tests showed that the grate area was much larger than required (for this engine, at least) and had to be reduced by some ten square feet to avoid wasteful coal consumption. The cylinders, although smaller that they would appear at first glance, due to the steam jackets, were also too large, making the engine a poor starter in less than optimum conditions. The sand consumption was the highest of any locomotive on the entire RSR. On the positive side, the tandem outside cylinders allowed an optimum cylinder volume ratio, which was finally determined at 1:2.35, the steaming was excellent, due to the large, wide firebox (which was in turn due to the placing of the trailing axle), and the coal and water consumption were very economical. Other features adopted from Chapelon's 160A were intermediate superheat and thermic syphons in the firebox.

Altogether, the trials and experiments lasted all of six years and provided invaluable information for the design of the now-legendary 564B of 1975. The opportunity was taken, on some of the engine's many visits to the works, of applying various liveries in an attempt to break finally with the almost classical black with red wheels which had been in force since 1931.

In 1968, no. 364A.001 was (finally?) fitted with poppet valves, which she also retained when rebuilt in 1980.