This is the medium sized rail in the BOM, which allows the tool head to move forward and backward.

Zesty Tip
When dealing with linear rails, it's important to not drop the balls! Take caution and put temporary end stops on the rail.

And, rails are not pre-lubricated from the factory. The factory oil is primarily intended to prevent corrosion and should be removed using alcohol. After cleaning, lubricate the rails with Teflon or lithium grease for optimal performance.