Could be.... someone could have found them before and slowly emptied them of gold so as to not notify anyone that they were found.
Otherwise, it does not make much sense to go through the hassle of buying trains unless there is something in them you wanted to hide. Being the rumors of gold for all these years it makes the most sense. I am also assuming the two guys that say that they found it actually already explored it before notifying people about their find.
I'm saying there might be trains with gold- or there might just be trains. they had armored trains- they may have decided to hide one/some while retreating, just in case they made a new offensive later. the gold part is cool- but I don't think has to be the only possibility for finding a train in the tunnels.
As fast as the German front was collapsing on the Eastern front, I doubt that they would have worried about hiding armored trains (which would be fairly useless for an offensive or even defensive tactics) just for the sake of the armored trains. If they were hidden, they were hidden because the trains had something of worth in them.
There are other instances of the Germans attempting to hide valuables as they were losing the war.
Gold or some other valuable is the most plausible answer in my mind. But real life is often weirder than fiction so who knows?