This is a short tale that may be useful to builders of locomotives of any size, not just 7 1/4 inch 'minimal' gauge garden railways like Karen's Little Railway. Having recently built a battery-electric locomotive named 'Minx', I was disappointed to find that when pulling a load it appeared to get wheelspin as soon there was any more than a slight gradient and soon came to a halt. I was puzzled as to why this was happening because the locomotive was designed so that the driver's weight was central between the wheels, and my bulk plus the weight of the loco should have been enough to give sufficient adhesion. I had thought the wheelbase might be rather long for our fairly tight curves, but my research had suggested that with our generous gauge widening (on all but the straightest bits of track), it shouldn't be a problem. I tried various solutions, but nothing seemed to make any difference...
Since my design was based on the drive layout of Colin Edmondson's 'Scamp' (and related locomotive designs), I looked for, and finally found an answer in the Facebook 'Scamp Locomotive Group', which can be found at   / 449895411876067   Thanks to them, our Minx is now a 'really useful engine'.