The floor is not a good candidate for a height reference point as it's absolute height varies from the lowest point (and floor drain) behind and to the left of the laser to the highest point at the back right corner of the room this shot looks into. This difference is not accidental - it allows for drainage of the floor.

To get consistent elevations for the railroad, there is one reference mark on the end of the concrete dividing wall to the left that serves as the "home" data point. All other elevations are measured from that point.