Photos show that the NB rail crossing sign was swept away by rear trailer, but the WB stop sign and lamppost is still standing.
Examination of more photos show the NB rail crossing sign is still in place (or replaced). More likely the crossing sign West of the intersection is the one that was swept away, perhaps caught in the connection between the two trailers, or propelled by impact.
The lower front section of the bus is sitting just East of the tractor. Various photos show each front wheel seemingly attached to its axle.
The driver's side window is still attached to the upper bus body.
The truck driver seems not to have been seriously injured physically, but mentally he is not doing well. That makes it unlikely the collision point was at or near the cab, which seems to have suffered little damage.
I suspect the bus first impacted the first trailer ahead of the rear axles.
The skid marks at an apparent 45° indicate equal momentum vectors at right angles, but the tractor and two trailers make the momentum solution beyond my understanding of physics. However speeds would be an inverse ratio of whatever masses were in play.
Rollover is common going over the shoulder, but that the first trailer has twisted some 90° illustrates the severe forces at work.