I have had problems with two Hornby Javelin drive bogies derailing. We have a curve on an incline and at this point on the track the two Javelins have been derailing. After studying the power bogie several times I could see that the leading axle was lifting and this was causing the problem. If you have seen a motor cycle accelerate too fast the front wheel lifts, this is what was happening to the power bogie. The simple answer was to change the two axles over so you end up with the axle with traction tyres at the front of the bogie.