I would say your best bet with a single shot on each rail is to shoot them separate and stitch them. Easily doable with 3 flashes, unless you feel like lighting something in the background that would add to the shot.
Lights labeled 1 are your main lights, have them a stop brighter than the ones labeled 2. Toy with the angles and heights a bit to see what you like. Standard 3 point lighting for portraiture usually has the main light 45` off camera and 1 stop brighter than the fill light which is about 10` off to the opposite side of the camera than the main light.
Take the shot when your rider is directly between you and flash 3 on the lower rail. That way you will get a nice rim lighting and hopefully the rail will act as a gobo and you'll get a good looking shadow coming towards the camera. It may get distracting though.
I would have the flash #2 on the lower rail aimed up a bit, maybe with a flag or snoot so it doesn't wash out your shadow of the rail.
Try getting a long exposure before the flash to pick up the ambient light on the buildings.
I'll start with saying I've never tried this before, but in theory it should work depending on specs of the flash.
Do any of your flashes have a strobing feature? If so, set flash #2 to strobe at probably 8 times (however many "frames" you want) at about 8Hz. Then you can get a sequence with a single long exposure shot. You would probably have to move the flash back a bit and set it close to full power to light up the rider for the length of the entire rail. (I'm not sure if the recharge rate is quick enough for full power, and it may burn out your bulb. Check our manual.) Flashes 1 and 3 would only fire once to give you one shadow/background light. You would have to move flash #3 in your lower shot to be in line with where the rider would be for your first "frame". Otherwise you'll get a shot directly into the flash without the rider blocking it out.
Try it, post your results!
"when the government says its necessary to see your genitals, the terrorists have won." - geteducated
But still for a forum that has swayed so many internet contests, fuel tv should be a cake walk. Go go gadget newschoolers.