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One option could be a Moulton APB. Can get one off eBay for around (5-600). They can come with niggles though. Seat post keeps slipping (my solution was to get a specific collar for narrower posts, though I still needed to use a coke can shim), front suspension can seize (some plastic part expands and then can't move. Can be sorted by buying a new replacement part and swapping out), can rust from inside out (dont ride in the rain, or spray some of that rust saver stuff down the seat tube). Also, they are dead heavy.
They look awesome though.
And can be split in two. In case, you need to errrr take half a bike somewhere.
I am in winter land at the moment. Daily commuting in sub zero temperatures. IMO, these 20 inch (406) studded Schwalbes are by far the best option for snow and ice. Last year I was on 650 studded tyres, they were OK 90% of the time but I had some proper oh shit moments on black ice covered by dry snow. (I presume it has more to do with laws of physics than the tyre itself). With my Dahon I've had no issues. But therein lies another problem; the Dahon is fucking heavy and it looks shit.
My ideal set up with be a minivelo with 406 wheels, space for wide tires and either V-brakes or discs, and ideally it would look nice. I am tempted to order a custom Ti frame from XACD or similar and shift all the parts from the Dahon over to the new frame, but then I have no idea how I'd sort out the steerer tube and headset. Might also be more expensive than just buying an OTP minivelo – if only I could find one that fits the bill.