Re: I must say I'm surprised....
Castors! Fitting, that it sounds like a cuss word. Never met one that didn't eventually cause me trouble.
While Google works on the latest design revisions of its custom-built racks, beefing up the castor specs, no doubt, they could do better.
If the racks have wheels, I assume that one design goal must be quick and easy physical swapping/removal/addition of fully loaded modular rack assemblies.
Ditch the castors for something like a custom-built roller jack to raise and move the custom-built rack. Two mating jacks, front and back, joined by lifting bars inserted through the rack. Foolproof quick connect fittings for all cabling, fibre, cooling. Solid footing. How about a super-rack system - like a rack for racks, maybe, while you're planning your next 1000-rack deployment?