Rather than seeing this as a rail gun, I like to image it's more a a molecular pulse cannon. Don't ask for the science behind it
My idea is that it supercharges the electrons in the target so that they "spin out" over-accelerating to the point the structure of the element breaks down and melts/erodes. It can bore through steel and concrete like butter. Woe to the infantryman who stumbles into the nearly invisible beam because his helmet would instantly fuse with his super-heated skull in the same millisecond that his brain boiled and erupted out his face. Eww!