Lengths are not that important, except in situations to meet overall combination lengths.
An A train has a convertor dolley used to connect the first trailer to the second trailer.
These are extensively used by most U.S. LTL carriers - they enjoy the equipment flexibility offered.
Trailers are often 34 to 28 feet in length, but can also be a combination of a "long box" of 48 feet or so and a following 24/28 foot trailer which is a "Rocky Mountain Double" but still using a converter dolley.
A Btrain can be made of 2 different length trailers, the main difference being that the last trailer, or full trailer to some and "kite" to some of those guys running LTL rides on an extension of the lead trailer which carries it's own fifth wheel.
In the case of B Train vans, the extension is on a sliding bogie that slides under the lead trailer for single trailer use and slides out for B Train use.
Nationality, as mentioned, has nothing to do with type.
Canadian carriers favour the B Train for the simple reason of 1 less pivot point to be concerned about.
U.S. carriers have invested alot of money in A Train technology.
If you go back to Hank's main picture site, Martin Phippard has extensively photographed and written about the development and use of both A and B Trains in world-wide service.