Each half axle is pushed straight into the sleeve on the pinion gear, but it is a very tight fit. The fit is so tight it is not unusual to see a split in the gear sleeve. Unfortunately that will allow the axle and wheel to slip under load. Over time the axle will work its way out of gauge, wobble and cause derailments. If you can turn a wheel with your fingers, the gear sleeve is split.
I repair mine with monofilament fishing line and Super Glue. When replacing the axles, there is a stop in the sleeve so the wheels are evenly spaced and properly gauged.