Tank engine.
The Px48 isn't that far off the prototype- it's not a bad representation, but to me it's close enough to be obvious what it's trying to be, without being close enough to be right, so it looks wrong. I think that makes sense...
The tank engine is clearly not the 0-10-0, nor is it pretending to be. Instead, it looks like a little sister to it- there is a definite family thing going on, it's a very viable "might have been", and so to my eyes at least it looks right. I think it fits better with the rest of you fleet as well.
And as NHN says, it's also a hell of a lot nicer to look at...