Builders working at Birmingham New Street Station found the letters inside the postbox when they removed it last week.
Strangely, the box could easily be seen in front of thousands of visitors to the station for over two decades, The Daily Telegraph reports.
Royal Mail is now attempting to deliver the letters to their planned destinations, but have been unable to explain why the post had been ignored.
The box had been sealed earlier this year after travellers had complained that their fingers kept getting caught in its small flap. An 'out of action' sign was later stuck on it.
A postal worker said: "We didn't know it was there, to be honest. It was unbelievable to find the thing stuffed with old letters."
A Network Rail spokesperson said: "For security reasons many mailboxes have been removed from stations but presumably it seems that the post in this one was simply missed.
"There were letters that were meant for Australia and America and postcards to people's friends and family in there, just lying under a thick layer of dust."
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A railway spokesman added: "We couldn't believe it would be missed by anyone. But with the greatest will in the world we're not in the mail delivery business."
Finally, a spokesperson for Royal Mail said of the postbox: "We would not seal the box with letters still in it.
"We believe customers continued to post letters, oblivious to the fact it was not being emptied."
A similar incident occurred in Poland in 2009, when a postbox had been discovered unemptied for five years.