And the feet must go up and down, don't stop

Mono, okay, but up and down? People on stairs, elevators, ladders? Or something completely different.
I cycled to Dieburg today, expecting to see some stairs between the platforms of the railway station. But on my way there I noticed that my feet and lower legs were already going UP and DOWN, all the time. At 70 rpm in average, my preferred cadence.
So I took my mobile and shot some pictures, illegally, during my ride.

Thanks to SueJay50 for hosting today's Mono Monday challenge.