Yet another use for 4020 rail; A 3D Printable similar to "Rope Cam", "cable cam", or other similar trade names.

Includes Fusion 360 Original Files for editing if so desired.

)Belt = 3M-300 or 3M-315 (GT2-3M x 300) @ 6mm wide (HTD3M) - search reprap
) 608 Bearings - 2 Per Wheel
) 3D Printable pulleys, gears, motor mounts, ESC holder and battery box.
) Mount to a 1M or "whatever length you want" 4020 extrusion
) Get a 3665 or 36XX motor kit with ESC - First two digits = diameter, last two = length (longer = more power)
) Get a receiver. I like the Flysky or Frsky 6 channel radios.
) Bind it to your transmitter.
) Charge up the battery - I used the Tattu 1550mAh 3S 11.1V
) Use at your own risk. Program your stops and lower your max throttle curves and do a radio dropout test to your fail safe settings to avoid a runaway
) Use a rubber band on the drive wheel to get traction on the line
) A heavier load requires a heavier line, which itself is heavier... Learn about loading, and use at your own risk, not above people or property at risk.
) Definitely add a "Safety hook" which would capture the line if the wheels went off track.
) The tightness vs slack of the line matters, and a less elastic line is better. (one that doesn't stretch along the length much)
) Able to mount and dismount the rig without disconnecting the line. Add a capture hook line for extra safety
) Add a counterbalance or outriggers to cancel harmonics from the wire length and balance the moment arm of the rig.
) Bolt on whatever you want, lights, multiple cameras, etc..
) Consider running two parallel lines, and then print another pulley to ride on it. Use two identical spacers to get the lines straight across your span.

Tip, when you bolt on the axles, make sure to leave room to not compress them! Use two nuts on the outside, and check bolts as a part of your pre-flight routine.

Then add lights, camera, action, and you have a cable cam... boom.

Also may be used with time lapse if you can program your motor to go slowly, probably with an entirely different motor, like a low ratio geared dc motor and a stop limit switch on the rope.