The Safety Pin by J.S. Fletcher

All unconscious that Simmons Hackdale was on the same train and only a few yards away along its corridor, Mellapont went back to Southernstowe, plunged in thought. He had an instinctive feeling that Bartlett had told him the truth and revealed all that he knew. What did it come to? In Mellapont's opinion, to this--that if there were any people who knew more about the sand pit murder than anybody else, these people were Mrs.

Champernowne and John Hackdale. Obviously John Hackdale was hand-in-glove with his employer in the effort to silence Bartlett.