According to the the UK Government’s Cabinet Office:

Top secret – The compromise of this information or material would be likely: to threaten directly the internal stability of the UK or friendly countries; to lead directly to widespread loss of life; to cause exceptionally grave damage to the effectiveness or security of UK or allied forces or to the continuing effectiveness of extremely valuable security or intelligence operations; to cause exceptionally grave damage to relations with friendly Governments; to cause severe long-term damage to the UK economy.

So it’s not very clever to have one of your employees leave such a document on a train..

Our correspondent said that across several departments in Whitehall on Wednesday evening there is said to be “horror” that top-secret documents could have been so casually mislaid.

I’ll say! Apparently the culprit has been suspended from his job.

Update: It happened again, though this time it’s not as clear what the actual classification was.