That IS the critical question Uart. Thank you for asking it. Yes it matters greatly how the cart is configured. Mender, Spork et all go to great pains to emphasize that the reference frames are equivalent. Indeed, the frames themselves are equivalent. But the cart or boat or whatever is IN the frame must also be exactly the same in both frames. You can’t have a sail boat in one frame and a motor boat in the other, for example. ALL the evidence for DDWFTTW is based on a cart advancing on a treadmill. Then that evidence is used to prove that a cart can advance DDWFTTW. There is no evidence to support that claim other than the treadmill evidence. As I said, with a prop of a certain pitch, a cart can advance on a treadmill. In order for anyone to use that evidence that it can also go down wind, the prop pitch must be exactly the same as on the treadmill. If the pitch is changed to go down wind, the treadmill evidence means nothing as you are essentially comparing two different machines. Please consider that carefully.