Ok, tried it. Got the thing running, as described in the previous post. For the moment I just used the Pennsy H3 skin, because I was already using that for wheels and drivetrain components. For this test I just wanted something that looked like a locomotive, so the skinning is a bit of a mash-up right now, but it gives the general idea.
Quick_test_new_geometry.jpg
With the geometry set up this way there is good news and bad news. The good news is that behaviour over undulating track is now very good. It doesn't look stupid, and behaves much like any other loco/tender combination. The other good news is that without trying it looks more or less decent for skinning, so as long as I don't go OCD looking for perfection I can skin monster Garratts to a reasonable standard. So, monster Garratts are feasible game assets.
The bad news is that, for the front and middle units, the behaviour around corners is not as good as I would like. It's looking like the best compromise will be something that doesn't screw with the game engine's expectations quite so much. So, it'll probably end up with the front tank/bogies/etc coded as Truck1, and with the body file geometry (if it has any geometry) centred on Y=0. This will make it as good as the double cargo cars round corners, so pretty good anyway.
Edit: Just re-jigged it to use that geometry (all front unit bits as Truck1). It's better for behaviour. Zip attached.
The "tender" (rear unit) is fine around corners, except for the usual RT3 game bug of scrunching around right turns and stretching out around left turns. This applies to any loco/tender combination, or to any cargo cars in the consist. It's not ideal, and is why I'd prefer to make smaller Garratts without using the tender slot for the rear unit. However, if monster Garratts are wanted (and I definitely want them) then using the tender slot for the rear unit is going to be the best compromise for all-round decent behaviour.
The other thing I found out is that you can't have smoke coming from trucks. You can have steam and lights on trucks, but apparently not smoke. No idea why they'd screw us on smoke, but they did. Fortunately for this Garratt it doesn't really matter, because the chimney ends up close to the Y=0 point anyway, so the smoke file can be attached to the body file and it still works ok.
Anyway, the current rough beta is attached if anyone wants to mess around with it. Note that this does have monster mash skinning and very provisional stats, but it runs around tracks more or less like it should. If you are going to try it out, I recommend removing any files for
the old AD60 basic beta if you have those installed.