For the fourth consecutive year, the C train is at the bottom of its class, according to the annual Straphangers Campaign “State of the Subways” report card, which counts the rickety line as both the dirtiest and most likely to have mechanical delays. On the other end is the Q train — reliable and pristine, relatively — which is good at showing up on time, keeping riders informed with announcements, and doesn’t sound as if it’s about to screech and rumble right down through the tracks into hell.