Long term readers might recall that the H&DLR a couple of Saxon brake vans, built to scale by Herr Hilbert from his kits for me back in 2013 and 2015. I'm very fond of them, but one at least tends to suffer from the axles falling out of the bogies. The axles are long, but the bogie side frames are also set quite far apart - just enough for an axle to drop out occasionally. Finally found a possible solution - adding a few 3mm or 4BA washers to reduce the slop. It took a few washers (but a spare oversize nut was too thick). 

And a short video of the two vans under test with a couple of Rollwagen.