This is a cable car in San Francisco, the only USA National Landmark that moves. While I was taking my ride in one from the Powell street BART station to Fisherman's Wharf, I heard one of the brake-men explaining the technology to another train rider. There is a cable running underneath the street that is always in motion. The driver engages a clamp that grabs the cable when he wants the car to move forward and disengages the clamp when he wants to stop (braking where necessary and with all the hills in San Francisco, that is pretty much everytime he wants to stop). While walking any of the streets that still have cable car runs, one can hear the moving cable as a constant hum underneath you and it took me awhile to figure out what that rumble was.
To me, this is a somewhat rough but amazing picture of the Spirit of God. He is always moving and if you know what to listen for, you can hear his rumbling and humming, but you can't see him. However, when someone opens their life and hands and grabs onto Him and what He is doing, they start to move, they are propelled forward, and on the surface, it looks like they have developed great power to climb mountains and overcome obstacles, but they have merely attached themselves to a mighty God - there is no self-propulsion involved.
Look down, go deep, listen for the Spirit, reach out, and hold on for the ride of your life.