Don't mean to be critical, but with your laser level and a rod, leveling the floor AMAP would discourage spotted, heavy railcars from accidentally creeping into the slightly lower back wall, without wheel stops, done before door work was my concern.
Bro and I leveled my sloping Sandia foothills back yard with an OLD borrowed Gurley level and a home-made rod, driving small hubs every ten feet square. Worked fine.
Shoreline Trolley had their GC level two new, higher car barn sites before the metal structures went up, then used a Bobcat and a small rail hopper/dump push car to spread interior ballast.
Good luck.