I'm afraid that the fixed pony truck with flangeless wheels was a step too far for me; the flanged wheels derailed the loco even on Peco medium radius points.
So - I set about modifying the chassis to a conventional swivelling pony truck arrangement.
Firstly, I detached the insulated tender couping bar by unscrewing it. I took care throughout what followed not to damage the power leads to this bar.
Then I cut, drilled and secured a strip of 6mm. x 0.25mm. brass between the pony truck frame fixing bar and the portion of the chassis which has the slot for the trailing axle. I used the fixing screw hole for the axle retaining plate with a longer self-tapping screw, ( the rearmost of the two pony truck fixing screws), and a 10 BA CSK screw in a tapped hole drilled into the pony truck fixing bar. The strip of brass needed a slight joggle to suit the difference in level between the chassis block and the pony truck frame.
The brass strip could now be removed along with the axle retaining plate and the axle / wheels, and releasing the pony truck frame by removing the second self-tapping screw.
I scribed the rear profile of the ashpan hopper castings onto the mainframe before removing the hopper castings after unscrewing them.
Next I clamped the chassis block upside-down in a vice, taking care not to damage the motor, and took a *deep* breath!!
Using a piercing saw, I cut along the scribed rear profile of the ashpan hoppers up to the top line of the hopper castings, then horizontally, stopping well short of the cast 'prongs' at the rear of the chassis as I did not wish to sever them from the chassis block. Finally, I cut vertically down to the end of the horizontal cut to release the section of the frame casting which carries the trailing axle.
I dressed back the cut faces of the released section of casing with coarse and then fine files to create working clearances, to allow the reassemble pony truck to swivel.
The radiused corners between the pony truck outside frames and the fixing bar were filed back, also to allow clearance for swivelling.
I tapped the forward pony truck fixing screw hole in the chassis 6BA; and slightly opened up the counter-bored recess in the forward fixing hole of the pony truck fixing bar.
The pony truck casting was reassembled with the separated section of chassis casting using the brass strip and at the same time the trailing axle / wheels and the axle retaining plate were fixed in place.
Using a 6BA shouldered screw, (mine is of Hornby Dublo / Wrenn origin), I attach the reassembled pony truck to the chassis via the forward fixing hole and checked that it would swivel.
I found that I needed to adjust clearances at certain points with a file but eventually arrived at a satisfactory situation. There is already loads of side-play in the trailing axle; I didn't need the pony truck to swivel a great deal.
I did need to crop a little off the under-cab piping where it turns in between the locomotive and tender drag-boxes, but this is invisible when the tender is connected. I also slightly cranked-out the metal support bracket for this piping to provide extra side-clearance.
After I'd shortened and reattached the loco / tender coupling bar, the coupled loco and tender negotiated Peco medium radius points without any derailment or binding; the rear pony truck following the curve in a prototypical manner.
A bit of a heart-stopping surgical exercise but well worth the effort in my opinion.
Regards, John Isherwood.