What was inside this tunnel, that killed 520 passengers?

On the rainy night of March 2nd, 1944, the overnight steam-power freight train 8017 from Battipaglia to Potenza, did not pass through the railway station at Balvano in southern Italy. The station master there was not overly worried. But later at 5:10 that morning, when one of the crewmen from the freight train staggered into the railway station, the Station Master knew something was seriously wrong.