And yet another minecart mystery. While in general, diagonally-moving carts don't give a fig about tracks they pass over, _sometimes_ they decide to re-join the normal tracks. I've only seen this happening on track corners (crossings, t-sections and straight track get ignored no matter what) and also only when the movement angle was a good way away from the balanced 45° angle. Accelerating the cart with a medium-speed roller and then diagonalising it with a highest-speed roller gave good results, the cart moved diagonally for two tiles and then settled into the corner and started following tracks again.
Currently, finding the correct angle and the correct track corners to catch the cart is very much hit-and-miss. It does provide a way to switch track via perpendicular roller without restraining walls (although that's an outmoded and crummy switching method anyway); more interestingly, there's no need for a 'pathable' track connection between the routes one switches between and it's quite possible to build a four-way split (i.e. turning the four possible combinations of two binary inputs into four distinct outputs) with the minimum two rollers.
Oh, and building powered machinery in a glacier is expectedly tricky, everything wants to freeze solid and you have to put all machinery on constructed (can still be ice) floor. The windmills insisted on freezing, seems like the central tile at ground level still counted as too-cold natural floor, even though it was empty space. Got them unstuck by dismantling and re-building a connected but unfrozen gear assembly. It's actually kind of amusing when a phantom problem can be solved by voodoo troubleshooting.