I think I know why B-doubles often have tri axles.

Weight might have more to do with it....

Backing is a little easier because there is one less pivot point that a doubles set up with dolly.......

 

Lmao. What Cat said, it is all about weight. lol

 

Them B-doubles got your interest, huh?

 

Funny thing is, if you look at the weight laws (here in the part of Canada I'm in anyways) the load per axle actually goes down with each axle added to a group. Single, non-steer axle is 9,100 kg, tandem axle group is 17,000 kg and a tridem group is typically good for 24,000 kg. With that in mind, I think the weight is irrelevent. Its the lack of converter dolly, which in essence is really just a super short trailer. The more pivot points you add, the more the combination wants to wag.

 

It is probably worse than that here, we are allowed 38,000 on a tandem, and just 42 on a tridem, between that and the bridge length, a b-train that is Canada legal on weight can not come across the border in to Alaska.
There are a few, not many side dumps here set up like b trains, but they have 4 axle leads and both trailers are full length. It is rare to see a set of them anymore, the one company that ran them went out several years ago.

 

I have Super-B train (B-Double) hopper bottoms.
I’ve even backed them into a mechanics bay without room to pull completely straight ahead. You get use to it.
Fun fact. Sometimes when I call a farmer that I’m going to load at, the first thing they ask is “can you back up ?”. Some of the older farm yards are pretty tight.

 

First set of Super B's I ever pulled, taken in 1978 at the Moosomin SK scale.
Not often I had to split them, could back them nearly anywhere you could put a straight trailer, but trucks were a bit shorter back then.

The GCVW at the time was 135,000 lbs in Ontario, 110,000 lbs in western Canada which was later raised, also why the lead axle is raised.