The thread will will pass over curve in the brass guide to make the 90 degree turn and down the tube exiting under the layout. Be sure there are no metal burrs left on the tube after filing that may cause ware on the thread. I used white thread in the photos for demonstration, but a gray thread was used and hard to see. Pulling the thread across a wax candle before installing will give some sliding lubrication for the Thread Guide.
Note: By removing the top half of the brass tube, it will be easier to run the thread down the tube.
Under the layout various means can be employed to operate the thread for pulling it both ways. Ocean type fishing weights are added to each end of the thread. A simple crank that uses a 1/8 inch dia. wooden dowel with the thread is wrapped around several times for added friction. Or a slow motion geared down motor with a forward and reverse push buttons on the edge of the layout. Right now I am using ocean fishing weights for counterweights at both thread ends hanging below the layout.