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.