In this closer look at the Moffat Tunnel west portal note it is only single track which leads one to wonder if it's a bottleneck to today's traffic? The tunnel is 6.2 miles in length. To the right is the water supply pipe that feeds Denver. The lettering on that structure reads:  MOFFAT-WATER-TUNNEL. This smaller pioneer tunnel parallels the railway tunnel. Water began flowing through it in 1936.