They are, and for non-driven wheels, the bogey and train cars basically sit on top. There's only a few minor things like brake hardware that need to be removed

They are, and for non-driven wheels, the bogey and train cars basically sit on top. There's only a few minor things like brake hardware that need to be removed to remove a wheel. When there is a derailment, many of the wheel sets fall off. The axles are tough. Each axle weighs about 1 ton if I remember correctly. Each wheel can be reworked on a lathe several times (either with the wheel set removed or in situ on a drive-through floor-mounted lathe). After a few years, the diameter of the wheel is out of spec, and new ones are pressed on the axle. Axles can last about 75 years.