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?
Unlikely. You would still be accelerating your mass from zero, and you’d have plenty of time to stop pedaling. If you did it would be an extremely low-speed crash.
I’d say that riding off the side would be pretty likely to cause a loss of control though.
so still might want to wear a helmet, at least if you’re not very experienced
That would be wise, as well as having an assistant or a wall/doorway nearby to steady you.