The French train operator SNCF has discovered that 2,000 new trains it ordered at a cost of 15bn euros ($20.5bn; £12.1bn) are too wide for many regional platforms.
The BBC's Christian Fraser in Paris says that it is an embarrassing blunder that has so far cost the rail operator over 50m euros ($68.4m; £40.6m).
Our correspondent says that the cost is likely to rise even further.
Construction work has already started to reconfigure station platforms.
The work will allow new trains room to pass through. But officials say that there are still 1,000 platforms to be adjusted.
* Old wood working adage, "measure twice, cut once."