A major problem, if not the defining problem, of steam locomotive design is the balancing of the reciprocating masses, which, as we have said before, hangs over everything like the speed of light over space travel. The RSR design team at Akohniçe tried several times to solve this conundrum, producing in the process the ill-fated 665A, 655C, 655CC and 655CD. All of these efforts were thwarted not by the balancing issue but rather by the more mundane one of accommodating the cylinders at the rear end. In the USA both the Pennsylvania and Baltimore and Ohio Railroads produced duplex locomotives with rear-facing cylinders, not to forget the Delware and Hudson's no. 1403. This feature usually found the cylinders in a cramped location beside the firebox, exposed to dirt and excessive heat, which played their own part in raising operating and maintenance costs. Add to this the direct restriction on the size of the cylinders and the difficulty is much greater in the confines of the European loading gauge.

The solution arrived at by Gorote's team finally allowed the rear overhang to be brought within reasonable limits while admittedly adding to the complication of the engine. In the 675A, the somewhat narrower firebox is in its usual place immediately behind the driving wheels. The rear cylinders are in a tandem arrangement much like that of the classical Doherty compound of the 1970s, giving them space alongside the firebox while staying within the loading gauge. The aggregate reciprocating masses are of similar magnitude to those of the 655C and its cousins and can certainly be balanced by those of the front cylinders. The space restriction has necessitated a much more complicated parallel drive for the poppet valves, with the inlet valves above the cylinders and the outlet ones below, the exhaust steam pipes passing under the ashpan. Artur Gorote, on hearing of the tandem rear cylinder arrangement, asked why nobody had thought of it before, to which somebody (just who, is not on record) replied that they had, namely Doherty himself in 1945.

Other departures from the 655C include a narrower ashpan within the frames, which have been spread as far as possible, the placing of the air pumps one each side of the smokebox and a four-wheeled trailing bogie. With the reduced weight at the rear end, thanks to the shorter boiler and narrower firebox, two carrying axles were considered sufficient. The arrangement of the poppet valve gear for the rear cylinders gave rise to severe misgivings and led to their replacement with piston valves, those above the cylinders being for admission and those below for exhaust.

Although construction of the engine was authorised in March 2009, actual work was not scheduled to start until the summer and finally completed in October. Trials of the 675A bore out the optimism and confidence of Gorote and his team. On the notorious Rovniebera Bank, the prototype locomotive has already astonished observers with its haulage power, speed and efficiency. A load of 5800 tonnes was hauled steadily at 120 kmh, a total output of 6100 bhp, truly a record on European metals.

Text and graphics © Norman Clubb 2009