Sorry about that - I'd managed to remember that they were on a line isolated from the main system, but not that that was in Malaysia apparently. Possibly because I mentally associate Malay expresses with poppet valve three cylinder pacifics.
Their YD heritage is interesting, I don't think any YD's, or even related classes, ran in Malaysia, so the commonality was presumably for the benefit of Vulcan (who had build quite a few YD previously) in producing a derivative design quickly/cheaply, rather than the railway, who were going to wind up with a class of three anyway.
This I think is doable by shortening the wheelbase - the existing valve gear etc can then be relocated to the third axle, bring the cylinders up to, but clear of the leading driver. All at the cost of new frames and coupling rods. I've seen it done, but not attempted it myself. As yet, at least.
In terms of size, I've got a gut feeling that the IR metre gauge loading gauge is generally 11'3" tall, though obviously these aren't IR locos, so might not comply, they're probably a bit more complicated than just a YD with fur and a bit feet slicing out of the middle. If 11'3" is right, puts them at 143mm tall in 1/24th, which is probably a little too small compared to other G scale stock to give the right impression. WIth will be 8ftsomething, so probably OK anyway. The equivalent in 1/22.5 is a touch under 152mm (same height as the 1095 diesel), which smell more right, though possibly they'd then be a bit wide.
I should probably try an get some drawings.
"Dear Mr Fifteen,
"Please stop modelling all the locos I've got my eye on.
"Agrieved,
"Tyneside"
. . .
Does your flyingfifteen name by any chance related to a certain Rhodesian/Zimbabwean locomotive type? That's on my list too.
Jonathan
g-bits.co.uk