Gman suggests the bomb didn't go off on the bus. The limited damage and loss of life (2 fatalities I think) was a result of the detonator going off in a bungled bombing. Leaving a bomb and maybe a bomber for the Police.

I comment at Gman that the bus shows no sign of 5kg or 5lbs of high explosive, but the trains do. Trains are designed for much higher dynamic forces and impacts than cars, trucks and buses, because of the much higher dynamic forces in a collision. A survivable impact by 500 ton trains colliding at 120km/hr is probably the design limit - they need to be built strongly. Yet, the trains show significant explosive damage - admittedly to the sides where they are less strong, but there were eye witness accounts of flames and certainly the soot and burning could be seen on the victims; of which there is no sign as Gman notes on the bus.

Gman's analysis has not been picked up CT yet - he should standby for the call.

(Double Hat Tip and a bow for Gman)