Aha, its a
Heisler.
That has a two-cylinder "twin-vee" engine amidships, driving the two sets of wheels through driveshafts. The cylinders really hang out (as in the machine we are considering).
Here is a pic from Wikipedia of a smaller one:
Here we see the opposite cylinder in more detail:
More scoop here:
http://en.wikipedia.org/wiki/Heisler_locomotive
More scoop here:
http://www.gearedsteam.com/heisler/heisler.htm
This is more like "ours":
The rear driveshaft seems to be gone.
In this kind, the "truck" under the tender is driven too (can't tell whether "ours" originally had that or not, but probably).
Ordinarily, only one axle of each "truck" is driven by the drive shaft, the other two wheels on the truck being driven by side rods from the first axle. ("Ours" may be that way too - can't see in either shot of it I have.)
Carla says, "They can run but they can't hide".
Nice catch, Stuart.
Best regards,
Doug