OK, here’s how North American multiples are defined:
Doubles: 2 28’ single axle trailers, the “kite” (last) trailer connected by a single axle A-train dolly (a pintle hook connector). Max weight: 80,000 lbs
Rocky Mountain doubles: Lead trailer is a 2 axle 48’ (or longer) trailer, the kite trailer is a 28’, hooked to an A-train dolly. Max weight: up to 120,000 lbs
Turnpike doubles: 2 48’ 2-axle trailers, hooked with a single or double axle A-train dolly. Kansas and Oklahoma allow 53’ and even 57’ trailers hooked double. These units are only seen on toll roads in NY, MA, OH, IN, KS, OK. Max weight: 144,000 lbs. Also seen in Quebec on Autoroutes, but often are 4 axle trailers , max weight: 220,000 lbs
Michigan train/doubles: 11 axles total, in many combinations, from an 8 axle single (5 down, 3 drop), to A-Train or B-train doubles. Used for hauling steel mostly, only allowed in Michigan, and a few roads in Indiana leading to the mills in Gary, and to Cleveland. Max weight: 174,000 lbs
Triples: 3 28’ A-train hookups, all single axle trailers, max weight about 114,000 lbs. Allowed only in states of OR, MT, ND, SD, KS, CO, NV, and toll roads in OH and IN.
Canadian B-Trains: Many combinations, common flat bed version has 9 axles total, max weight 60 tonnes, all sorts of variations, incuding:
flatbed: 4 axle lead, 2 axle trailing
container: 20’ on lead, 40’ on trailing
There are many more, but that’s all I can think of right now.