Yelrow
The main problem with the Garret on points is due to the front steps. Mine tends to derail on point sets or curved points that are positioned in the curve and configured to move to the inner track. The rear wheel linkage presses against the steps which then pushes the tender outwards and forces the front pair of driving wheels off the rails; the pony truck has a lot more movement and thus is not affected.
I say 'sets' because the Garratt is designed for 2nd radius curves and thus makes the first point OK however, it is the combination of two as the front tender wants to move one way and the rear another as one will be on a LH 'curve' and the other a 'right'. This has the effect of acting like a tighter curve than the actual second radius and thus the steps come in contact. Similar for the curved points (when displaced from a straight entry) though this is because the loco is already in a curve and moving onto the switched part for the inner track again makes it effectively a tighter than radius two curve and the steps touch.
Unfortunately the steps are part of the detail and cannot be moved back a bit though they are Curved inwards (I assume the rear things were???). I will be looking to 'Straighten' mine which should not look out of place (especially if I also match the rear steps to the same profile), hopefully the extra 0.5mm or so will be enough to keep things on track. It is also possible to 'shave' a little of the plastic off the back of the step which would add that little more clearance. Thinking on I may go for this option first as it will the 'look'...
Now on to running backwards, the rear step are well displaced from the coal tender and so do not catch whilst the water tender when trailing will still catch the steps however, the weight over the leading wheels of the loco weight keeps everything in line.
Joe