Span bolster

A span bolster, in rail terminology, is a beam or frame used to link two trucks (US) or bogies (UK) so that they can be articulated together and be joined to the locomotive or railroad car at one rotating mounting point.

In effect, they make one "super-truck" out of the two, while permitting each truck to move relative to the other.

The use of a span bolster is normally signified in the AAR wheel arrangement notation by a + sign; thus a locomotive with two span bolsters, each with two two-axle trucks (the most common arrangement) is a B+B-B+B.

Specialized Schnabel cars with up to 72 wheels have been built for carrying heavy items.

The use of span-bolster-connected trucks is essential to allow the load to be transported through curves.

Span bolsters can be seen at both ends of this diagram of the 14"/50 caliber railway gun .