Aveling's main business was making road going steam traction engines, as I mentioned at the start. They simply took their existing tractor design & modified it slightly for RR use.
The "wheel/axle centre tie rod" is a little strange. Their normal road tractors only drove on the rear axle of course, the front axle being steerable (no kingpins or Mac struts, the whole front axle beam turns on a steam tractor). The usual system with steam road tractors was that everything was attached to the boiler shell, there was no chassis or frames as such, unlike almost all RR steam locos. Therefore they may have thought it necessary to add extra rigidity to the wheel base, especially as both axles appear to be driven. Looking at the photos, & others of the same make, it appears that the whole thing was unsprung, unless there is some form of suspension hidden deep inside the beast.
Thanks Helmut - I never knew such things existed. Also a surprising number of traction engine makers who had a go at producing "traction engine locos".
I wonder how they got the axle weight v traction correct between the flanged rail wheels (guidance around curves) & the rubber drive tyres ?. What happened over undulating ground either side of the sleeper/tie ends ?. Floating sprung drive axle ?.