The space between cars reduces that to about five feet. A minimum headway of about ten minutes (tight for a heavy freight train) reduces the effective speed by about two thirds. When one throws in the headway for the rest of the freight that must be accommodated on the line (especially all those empty containers going to Western ports and hoppers carrying coal) I'd cut that number at least by half. So your analogy is more representative at a five foot tube moving eight miles per hour, which isn't much different than a pipeline. Yet the place where that number really drops is in transits through urban areas, switch yards, and loading/unloading for distribution into a refinery. Hence, all other costs not taken under consideration, I would doubt that a train system could move oil as fast as a pipeline. I also wonder how many refineries have the yard capacity and unloading facilities to handle that many tank cars as fast as they might otherwise arrive.
Seen a tank car lately? The current ones carry a 160,000# payload, and are way larger than five feet OD. And average system velocity (moving from source to destination) is at 21 MPH right now, for regular freight. For coal and oil shipments? Significantally faster, although I can’t say how much. The point is, until some pipelines are put down, rail is the mode of choice to move oil, which is already occuring.