What's the difference between a 4w and an 0-4-0?
Although the UK locomotive engineering industry regards any 4 wheeled (coupled) loco as an 0-4-0, enthusiasts have over the years tended to amend the wheel arrangement classification for all locos that did NOT have side rods to indicate more accurately the "type" of locomotive it was, much in the same way as they would specify a DM, a DH or a DE. In continental practice, of course, a rigid frame two axle loco would be classed as a "B", and again would not specify whether it was rod-coupled or otherwise.
In the case of Andrew's collection, there are three such locos, and all three utilise roller chains (the same type as used on bicycles or motor bikes, but substantially larger). "Pluto", the Planet diesel mechanical, has its final drive gearbox frame mounted within the cab, and a single "triplex" chain drives the rear axle, the axles then being coupled by a single "duplex" chain. "Charlie" iand "Cheedale" are both based on Sentinel practice - "Charlie" with its rear-mounted RF11 is a similar layout to "Pluto", but "Cheedale" has its gearbox in the centre driving each axle in an inverted "vee" form. Thomas Hill and Sentinel used 2.5" pitch duplex chains throughout.
Chain drives were maligned by those manufacturers that did not use them. The short fact is that they provide a constant torque, free from the "cyclical" variations that characterise side rod drives, and enable the axles to be freed from the restrictions of hornguides so that they may more readily cope with rough tracks and sharp curves. In fact, the arrangement of mounting the axleboxes on radius arms (and spring leaves simply bearing on the underside of the chassis) is forced on the designer by the need to accommodate the stretch that inevitably occurs as the chains wear; which incidentally means that the wheelbase dimensions quoted for any chain drive loco will be "nominal". Thomas Hill practice was to recommend that chains be renewed when they had stretched by 3%. Their 2.5" duplex chains had a minimum breaking strain of 190,000lbs, and this was proven (on paper and by repute, in real life) to mean that the chains were stronger in use than the locomotive axles.
The one disadvantage of chain drives is that they are exposed, and in needing lubrication, they become dirty and the locomotives become caked in oily muck anywhere in 'throwing range' of the chains. The mechanical adjustment of chains for tension was only required occasionally (and much simpler to carry out than certain manufacturers liked you to believe) but although 6w steam and diesel locos were built, the hassle of having to 'double adjust' (i.e. adjusting a chain between axles A and B would result in double the adjustment being required between B and C) means that they were generally avoided.
Of course, a 4w or 6w wheel arrangement may not only be indicative of chain drives. Thomas Hill built over 25 "4w" locos for the UK Ministry of Defence - 19 were chain drive, but the follow-on ones were shaft drive, i.e. each axle has a final drive gearbox and the torque is passed to them by propellor shafts. Shaft drive locos possess the same technical advantages of smooth drive as with chain drives, plus the benefits of not requiring adjustment and being much cleaner. Their first-cost is of course much higher. For completeness of this review, 6w locomotives in industry include the Rolls-Royce/Thomas Hill "Steelman" type whereas British Steel (Corus) and the NCB operate(d) 6w diesel electric locos where each axle has its own traction motor, and mechanical coupling was avoided in the same manner as the bogie of a "Co-Co" main line loco.