HO train I.D.?

I didn't think Mantua/Tyco made a 2-8-4.

Maybe it was converted from a 2-8-2.
That would explain the small appearance.

Yeah, Mantua made some big stuff before Tyco bought them out. I used to have a Mantua 2-8-4 when I was a kid. I thought about Rivarossi, but it doesn't have enough detail, IMO. Rivarossi is pretty high end. It might be AHM before Rivarossi took them over. Unless there's some markings on it somewhere, who knows?