Simplify the following text for a Grade 7 native English-speaking person.
Original: Railway building began at Derby Works in 1840 , when the North Midland Railway , the Midland Counties Railway and the Birmingham and Derby Railway set up engine sheds as part of their Tri Junct Station .
Simplified: The building began at Derby Works in 1840 . This was when the North Midland Railway , the Midland Counties Railway and the Birmingham and Derby Railway made engine sheds for their Tri Junct Station .