I suppose trails would be kind-of neat. I would just say to make them dark and gossamer-thin, so that they're really hard to spot without knowing what to look for.

"railgun slugs are jest pieces of hot metal, and they use no propellant that would leave a particle trail."

Let it be an artifact of your ship's camera. Why does your high-tech spacey computer simulators bother to paint a trail for this weapon?

"It would make dodging rails easier (you're almost always looking at the trajectory at an angle)."

Thankyou, the two most critical posters for helping to make this idea logical and "make sense".