Just north of Farringdon station, and to the east of Ray Street, there is a wide cutting bounded by another of those brick walls nicely calculated to be just too high for you to see over. So prop your bike against it, and stand on the down-pointing pedal crank. You are looking at the Ray Street Gridiron, a spectacular bridge in a cutting that carries the Metropolitan, the supposed Underground, over the Widened Lines (now Thameslink).