This is a video demonstrating the technique published in Model Railroader Magazine, January 1978. There are other YouTube videos on the same subject as well, in this video I try to present more specific details of the whole process. The multiple passes of the tool shown are to make sure all of the ballast is removed from the top of the ties to ensure a good prototypical appearance. While even Class I railroads may have some ballast sitting on the mainline ties, it looks best to keep that to a minimum for a great manicured look. My thanks to Mr. Ken who is modeling the UP out West.
Alternatively, some modelers have used an elongated funnel shaped tool (round funnel squished to make oval) fabricated from copper tubing and brass sheeting with success as well. That design may be easier to fabricate, whatever works best for you.