Not 100% familiar with Vim Magrails. I'd assume a porta-band would easily cut them . If you bought an 8" rail you could get 2- 4" rails with a clean cut.seems 6" is the smallest they sell and only in green, orange and blue. wonder if i can make my own 4" and 5"? seems id wind up with some extra magnets as the rail gets shorter with each Kerf from each cut.
