Does Birona's Timewarp double the movement of a Steam Tank?
The Steam Tank definitely has a Movement value, albeit a "Special" value that depends on the number of steam points used. If it had NO value, then it would be listed as "0" or "-". It's movement is normally 3"xS (where S is steam points). The FAQ also indicates that the Rune of Wrath and Ruin (which halves it's Movement) reduces the Steam Tank's Movement to 1.5"xS.
I think this is a pretty clear case that Timewarp would make the Movement 6"xS.