I've worked on quite a bit of rolling stock and I'll tell you how I load wheel bearings: Assemble hub and bearings onto spindle, install washer and castle nut, tighten castle nut while turning hub (this can be with wheel on or off), when castle nut is tight, back off until hub turns freely and cotter key can be inserted. If I have the wheel off, I then put it back on and check for freeplay by pulling and pushing on top and bottom of wheel. You should not be able to shake wheel with any freeplay. This method involves some "feel" rather than brute strength or visual input.
As was posted, a 1 7/16" socket will fit a 37mm nut. It is found most commonly in 3/4" drive size so if you only have a 1/2" drive set, an adaptor will be necessary.
Good luck with the bearings.