I'm surprised you all missed the true motive power: the mower self-propel pushes the tandem bike via roller and belt drive to the trailer wheels.
And, that the mower is a KUBOTA, with a diesel, natch.
(As a fourteen year old I had to walk the mile to my farthest customers of a lawn-mowing business, pushing both lawnmower and edger [that wouldn't follow the same track]. Mom wasn't firing off that 428-4V FORD for such a short trip. But, of course, we were in Texas. No one has ever seen a bicycle like that, here. That's some kinder Commernist thing, ain't it?).
1990 35' Silver Streak Sterling
; 9k GVWR.
2004 DODGE Cummins 305/555; 6-manual; 9k GVWR.
Hensley Arrow. 10-cpm solo, 18-cpm towing
Sold: Silver Streak Model 3411