It sounds like it was found in an uncharted, unfinished tunnel that was part of “Projekt Reise”, an effort using Soviet POW’s and Concentration Camp Jews as slave labor, to build a complex of underground factories.
IMO, that’s why no one ever found it after the war, the workers were all killed by the Nazi’s as the Soviets advanced.
Then the killers themselves, likely the only ones left who knew where the train was, were themselves probably killed by the advancing Soviets.
April 1945 was complete chaos, as the German Army fell apart.
Are you implying the possibility that the Ark of the Covenant could be onboard that train?