I think they actually say "rigid" wheel-base. That would be the bit over the farthest apart wheels that don't pivot. And measured to the middle of the wheel.
On diesels, it would be one truck. On steamers, the longest set of drivers (if more than one set), excluding the front and rear trucks. Some steamers had fixed rear trucks under the firebox, so they would have to be included, but this is not often modelled.
