6 tile wide modular weapon that fires gigantic slugs/scrap balls/whatever that should deal grievous damage and have a very big knockback effect on the enemy.
i figure since smaller weapon types have direct upgrades (cannon -> large cannon, small laser -> big laser) why shouldn't rails have an upgrade too.
this won't make rails obsolete, as this mass driver won't launch projectiles at anywhere near as fast a velocity as rails, and will probably be a more close range weapon to make sure your slugs actually hit, and it will be logistically expensive, needing a lot of crew to make sure it has as much uptime as possible. each section will need more power, and the launcher itself will have a large delay between shots (10s?).