Stationary bicycles need support to stop from falling over because they lack gyroscopic stabilization. Right?
If you ride a free bicycle on a treadmill (so both wheels are spinning) will it fall over or stay upright?
If you fall over on a treadmill, does it matter if this treadmill bicycle combo is on a train?
Treadmills aren’t really that smooth, so, despite the stabilization of gyroscopic motion, enough wobble would creep in from the slightly uneven surface to eventually topple the bicycle.
But it would probably stay upright for a short while, as long as the handlebars were somehow locked into a straight and forward position.