We’re kind of talking at cross purposes here. A real roundhouse has something we modelers do not, and that’s a set of Mark I, Mod 0 eyeballs down inside the loco, connected to a Mark V manipulator device via a logic circuit. That is, an engineer / hostler riding the loco who can tell where to stop.
The real issue with using a mechanical object is that you might derail your loco or get it stuck on the object, requiring the old five fingered intervention to set it right. To this end, unless you want to rig a camera inside either your loco or your roundhouse, a foam bumper would be a good solution. you could also just wire a sensor and indicator light to let you know when the front of the loco reached a certain spot on the track.