And as poor Doug may well not be up with the acronyms
the SDDG4, sometimes refereed to as a SD4, is a prototype Shaft Drive version of the DG4, itself a Double Geared improvement of the Super Sentinel. The DG's and everything earlier were chain drive.
They brought in the numbers in about 1927 when they brought out the DG6 6 wheeler.
Its an interesting thing, basically the DG cab layout with the boiler out front, but what is basically an engine like the later 'S' types but fitted the other way around. So 4 cylinder single acting engine, rather than a 2 cylinder double acting engine like the earlier waggons. Obviously with 2 gears as the DG and later S
I seem to remember reading that at least one of the prototypes (probably a 6 wheeler) had what was basically a DG engine turned through 90 degrees, very like how Fodens did shaft drive with the Speed wagons.
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