Letters are written in both a business and personal capacity to and from a wide variety of correspondents. The series contains letters from, or on behalf of various prominent railway individuals including George and Robert Stephenson, Edward and Joseph Pease, George and John Dixon, Michael Longridge, William Hunter, John Robson Kay, Thomas Wrigley, Thomas Cabry and includes one letter from the office of George Hudson. It also contains early correspondence from Robert Young (sr), an early engine driver. Personal letters are friendly and often religious discussing Methodist matters including one letter from John Wesley Hackworth whilst he was at school.

Earliest letters discuss work on the Etherley and Brusselton inclines on the Stockton and Darlington Railway, with inclines also being mentioned later on in the series. Further on in the series letters describe operational railway issues, for example the possibility that horses were beating off engines on the Stockton and Darlington line, the use of stationary engines instead of locomotives, the use of ropes. The use of bellows by George Stephenson is discussed in the 'blast pipe' letter (HACK/1/1/11). The letter is purported to show that Timothy Hackworth was responsible for this invention, as Stephenson was still using bellows at this time. This letter is frequently discussed later in the archive by members of the family and other interested parties.

Letters from 1829 include letters from John Urpeth Rastrick and James Walker who compiled a report assessing the merits of locomotive power as opposed to horses. Letters from October of the same year describe the Rainhill Trails and discuss the performance of Timothy Hackworth’s Sanspareil. Letters also describe the Sanspareil’s performance including a letter from Alfred Kitching that describes some of George Stephenson’s unsuccessful locomotives.

1841 Timothy Hackworth moves out of the Stockton and Darlington workshops to concentrate on his own workshops at Soho Works, Shildon. By 1847 letters are evidence of Timothy Hackworth’s work for the London and Brighton Railway. In 1849 correspondence shows Timothy Hackworth unsuccessful attempt to sell his locomotive 'Sanspareil 2', including correspondence with J.E. McConnell of the London North Western Railway.