They measured the new trains, they checked against the latest specs on the book. But engineers at the French railway network forgot to go and measure the actual distance between lines and platforms, and it will cost €50 million (HK$531 million) to fix the problem.
France's secretary of state for transport, Frederic Cuvillier, called it a "tragically comical, mind-boggling" mix-up, blaming a lack of co-ordination between France's two state rail bodies, the SNCF and the RFF.
The rail companies acknowledged the embarrassing situation after it was revealed by satirical weekly Le Canard Enchaine.
Introducing "wider trains in response to the needs of the public requires us to modernise 1,300 of the 8,700 platforms in the French rail network", they said.
The problem affects 182 regional trains supplied by French manufacturer Alstom and 159 from Canada's Bombardier, which are due to come into service by 2016.
So far, 300 station platforms have been adapted since work began in 2013.
"It's a bit like buying a Ferrari that you want to fit into your garage, but then realising your garage isn't quite Ferrari-sized, because up until now you didn't own a Ferrari," RFF offered by way of analogy.