The "trainwheel" animation you show is pretty much the same motion that your leg and the crank use. The pivot mounted on the rear wheel is your ankle on the pedal and the pivot at the rear of the picture is your knee. The linkage changes nearly linear motion to circular motion in both cases.
The bar connecting the two wheels is just there to allow both wheels to share the drive load and traction. Connecting the two wheels on a bike this way is impractical since one (the front) has to be able to turn.