Both sides are known to lie. It might have been a false flag committed by the Ukraine against itself in order to blame Russia, but, Russia might well have collected missile fragments, especially including those with the serial numbers still conveniently legible, from Ukrainian attacks on its soil, and placed - and then conveniently “found” - those fragments in the train station after the attack. There is no good evidentiary chain of custody in military attacks, and both sides have strong incentives to lie.
By way of example, note that the FBI lied for 6 years that they never used any pyrotechnic devices during the Waco massacre (1993), but then admitted that they had. They also “lost” the door they claimed was “bowed out” by Davidians firing outwards, and they also claimed that the said spent pyrotechnic devices were actually illegal silencers possessed and used by the Davidians, partially justifying the attack.
SMDH.