There is a fair amount of intellectual property bound up in the history of blacksmiths, metal trades & metal tools. Files are hard & brittle, treat them wrong & they snap pretty easy. I keep one around mainly for sharpening engineer’s chalk which is harder than soft chalk & maintains a thin defined line of approx. 1 mm, something soft chalk just can’t practically do.
There is a way you can get a thin line with soft chalk though, it usually takes two people & a chalk line which is composed of strong string. You tension the line & rub it with the soft chalk bit by bit then you select where the line must go. Tension the chalk line between two points then one person flicks the tensioned string upwards it then proceeds to spring downwards leaving a chalk mark on the intended surface. Otherwise you may need a really long ruler which would be costly to purchase & difficult to store & transport.
a file photo
tension the chalked string line, then raise it a few inches & let it go.
the string flick leaves an impression on the surface