"Neither of them, however, could think of any simple, certain way of securing the money." They continued to discuss the matter, weighing pros and cons, suggesting methods only to pick flaws in them. "In March, 1924, the patient conceived the idea of securing"-- What? Thrill? The excitement? No. "Conceived the idea of securing the money by having it thrown off of a moving train...."
I used to think that the most impelling motive in life was passion, but in this case passion and a desire for revenge is swept aside for money. Money is the controlling motive in this case.