I've seen the "band" on the boiler before.
But what exactly going on with the paint, and that tender?
Is that a "rusty" rail crane, or artillery guns with camo 
That photo turned out pretty bad. It needs more light in the room, and time spent getting the distance right. More shots might help too. The pilot, the motor, the couplers, the tender trucks, etc., are often great clues.
As for knowing the gauge, it only takes a dollar 
...
Fold a bills length in quarters, and you have an approximate measurement of O gauge. Although slightly oversized, it just covers the rails 
