Bombproof bike stand.

It is indeed a one man bunker, a "Luftschutz-Splitterschutzzelle".
This bicycle way was formerly an industrial railroad track, the "Erzbahn", starting at the Rhein-Ruhr canal in Gelsenkirchen and transporting iron ore to the blast furnaces to the
Bochumer Verein. That kind of small shelters, big enough only for a man (or maybe two), were built to provide cover for those men working on or at the track in case of an air raid. The two I found still standing have been placed at at railroad switches.