A refrigerated train bearing remains of almost all of the 298 people killed on Malaysia Airlines Flight 17 was poised to leave rebel-held territory in eastern Ukraine Monday night, with the bodies to then be flown on to the Netherlands.

That grim conveyance, along with the handover of the plane's flight recorders from pro-Russia rebels to a Malaysian delegation, allowed the focus to tighten—both at the crash site and in international diplomacy—on establishing who brought the jetliner down on Thursday. (Follow the latest updates...