What is the best way to cut the k-rail?

Use a saw with a fine tooth blade such as a hack saw with a metal cutting blade. If you purchase the optional end caps the rough edge from sawing will not show.

You could cut it with any kind of saw

You can use a hand saw, low speed jig saw, Rockwell saws, dremal saws, dual saw, be sure to use a straight edge to insure a perfect cut unless you have a steady hand.

The rail is hard plastic so any saw should eat through it. Use a miter box for a nice square cut. I have cut it with my miter saw and table saw successfully.