An Eye for an Eye by William le Queux

Without a second's hesitation I rushed up the steps after Boyd, but on gaining the platform found that a train had just gone out, and was at that moment disappearing across the bridge over the Thames. The detective, known to the ticket-collector as a police-officer, had been allowed to pass the barrier, and had evidently caught the same train as Blain.

There was certainly an element of deepest mystery in the fact that the unknown man who had kept the appointment in St. James's Park, and had afterwards taken such elaborate precautions against being followed, should be revealed to be none other than the once purse-proud proprietor of Shenley. Quite apparent it was, too, that the object of Eva's visit to the park was to meet him clandestinely, for what reason was a profound enigma. The more I revolved the strange events within my mind, the more absolutely bewildering they become.