Surveyors working on the HS2 rail project lost confidential documents containing personal data of people whose homes are to be demolished.
Residents on the Shimmer estate in Mexborough, South Yorkshire, were told property valuation details were left on a train.
The rail firm is buying some of the newly-built houses to clear them to make way for the high-speed line.
The blunder was slammed by a local councillor as “unbelievable”.
HS2 Ltd apologised to residents for the “distress this has caused”.