Long distance trains tend to have one quiet coach - nowhere near 50% of the train. And it's rarely that much quieter unfortunately. Quiet coaches make sense if you're travelling for an hour or so where someone's refusal to turn music down etc could make a long journey unpleasant.

Quiet coaches on a 15 minute journey where the majority of people are fighting to get on the train let alone worry about where they stand so they can call the office to let them know they're stuck outside London Bridge probably aren't needed.