Tell me if I understand this right.  First, this kind of reminds me of my first statics classes in 1970/71.

The first truss is the steel casting that we can easily see, the outboard portion of the sideframe.  The second truss, then, is behind the first one and an integral part of the same casting but which integrates with the spring package and the bolster in a different way than the first.  You gotta hand it to 1920s marketing guys to come up with this term, but it does appear to do what the ad claims it does, namely support the spring pack in an additional and different way.  The curved shape at the top of the sideframe and bottom of the bolster adds to the flexibility in the transverse direction, it appears, and it may not really be part of that 'second truss' but a design improvement just the same.  And if it adds to the decreased point or line loading on the casting edges at different places, obviously a good thing.  

The Taylor truck with circular interface between sideframe and spring-bolster assembly is perhaps another attempt to solve the same problem, or at least one of them.

Ron Merrick