Re the Montague biframe; models were originally sold with montague and/or Schwinn or BMW branding on them in the USA
The image above shows a ~21" framed machine.
Rudge branding was used in the UK; this is a 23" frame (for 26" wheels)
If you take both wheels out the rear triangle swings inside the main frame triangle and (with a wheel stashed either side) this makes the whole bundle reasonably slim. A faster fold leaves the rear wheel in, and the front wheel out, but is a bit more bulky.
To allow the fold, the seat tube(s) are nested inside one another, are longer than normal and also have a slacker angle. To get the saddle in the right place (for most folk) the rail clamps usually need to be set forwards of the seat pin. Other than that all the cycle parts are utterly conventional; 7/8" dia quill stem, 1" threaded headset, 559 wheels, wide spaced canti bosses (so will accept cantis or Vs). There is one set of bottle braze-ons on the seat tube. The thing on the other side of the seat tube is a grease nipple, use of which helps stop the nesting seat tubes from seizing up. The front mech is on a braze-on fitting (because of the unusual seat tube angle) and is set for a 48T big ring. I think there is usually enough vertical adjustment to allow 46-50T at least.
Weaknesses are weight (two seat tubes are exactly twice the weight of just one), the gear cable routing around the BB is something of a water trap (there are short lengths of cable housing there, without which the fold wouldn't work). The BB shell is connected to the seat tube via a saddle weld; this means a cartridge BB with a large centre diameter will not always fit. I suspect the saddle weld will be the first place for the frame to crack in hard use, but I have yet to see this happen. [If it ever happens to my machine I shall weld repair it, and add some reinforcing gussets.] Local to me there are quite a few of these bikes being used as standard bikes, appearing not to have been folded for years.
Obviously any racks will impede the fold; I use a carradice saddlebag on an uplift; the bag and the uplift come away together so the fold is not impeded. My plan has always included mudguards which come away still attached to the wheels, but I have not yet (in quite a few years... ahem) fangled them, so my machine has mainly seen dry weather use to date.
Current Montague models are based on a later design and the range at one point included one with dropped bars and skinny 700C wheels, not heavy. Not cheap, but not a bad ride either. A bit like this;
the bike above is a custom one, not an off the peg model. But since all the cycle parts are standard, you can do your own build very easily. BTW with a frame of the newer sort , a padded sleeve around the front part of the frame helps stop the handlebars from swinging into the top tube. It also provides a neat cubby hole for storing odds and ends in; there is enough room inside for a mini-pump, tubes and a multi-tool.
cheers