The Doors Open — Michael Gilbert

One night on a commuter train, Paddy Yeatman-Carter sees a man attempting suicide. Intervening, he prevents the man from going through with it. However, the very next day the same man is found dead, and Paddy believes the circumstances to be extremely suspicious. Roping in his friend and lawyer, Nap Rumbold, he determines to discover the truth. They become increasingly suspicious of the dead man's employers: the Stalagmite Insurance Company, who appear to hire some very dangerous staff.