I propose pressurised, radiation-shielded trains with an oxygen supply. Crossing the edge involves many of the difficulties of going onto space, but never leaving the source means the journey can be made a lot more efficiently than leaving and re-entering atmosphere on earth.
Tracks with a gripping capability would be safer than free-roaming vehicles, as you don't want to be rolling downhill for several thousand kilometres in the event of an accident.
The tracks would get really high approaching the edge, so that the train could cross the edge perpendicular to the line to the center of gravity of the planet. No that causes more problems than it solves.
Maybe a carriage flipping mechanism at the very corner to set the carriage straight for the other side.
Regular wheels wouldn't work, you'd need some sort of toothed mechanism so they can really climb/hang as they travel.
Big disclaimer, I don't know crap about physics, but:
As you approach the corner, the gradient approaches 45% to horizontal.
Assuming earth-like volume, the atmosphere would be "bowled" in the center of the square sides, so it would be extremely cold, no atmosphere, at the edges. Imagine if Mt Everest was 100 times higher.
Easiest crossing would be at the equator, at the corners would be much harder - more climbing, more time outside the atmosphere.
Maybe the middle 2/3 of the cube would be habitable, with the outer 1/6, at the shortest distance across, being outside the atmosphere. As such, it would take a long time to figure out there's anything over the edge, never mind to try and cross it.
An adventurous type could use long-range remote controlled vehicles with recording equipment to find out though, and it would be hugely cheaper than a space probe, to get into action. There could even be an X-prize like "How far" competition, egging people on. The kind of people who make supersonic cars might be the likeliest candidates.