This is the test photo I took. One half of the Heljan Hymek bogie casing is on the sprue, and the standard spur gears are lined up. At the time it wasn't possible to buy the individual gears from Heljan or their service agents, but I worked out that they were 12-tooth, Mod 0.5, plastic spurs which were available on eBay from a stockist in Hong Kong. I bought quite a lot of them, and they are a perfect mesh with the original Heljan gears.
You should be able to see the new middle axle position which I drilled under the 2nd/3rd gear. It lies exactly on the scale 5'3" point between the outer axles.
It's so close to hitting the gear teeth that there's no room to allow the axle to rise. If it were possible to move the gear centres, life might be easier, but these are fixed by the manufacturer.