All very odd...surely if you have a straight track...ie East yorkshire, then that is furthest you'll see!...regarless of overbridges etc. OK it depends where you are standing (OP please define). Surely the curvature of the earth comes into play. I'm sure someone on here will make sense of the geometry of someone with eyes at 5 feet above platform...therefore 8 feet above rail level: the visibilty of a train roof on a straight stretch!!