Looks a nice epoxy job.
To fit the first rail I clamped a good wide straight edge on top of the beam then butted the Hiwin rail up to it, like Neale said about the machined step. Then I mounted the gantry and used the bearings themselves to line the second rail, just nipping the bolts up slightly. Before fully tightening the second rail down I put the straight edge up to it to make sure there were no kinks. After that I also checked the distrance between them at points along the length just too make sure they were parallel.