Light lunging really helps my thoroughbred gain weight, it also isn't just fat but nice toned muscle that forms. I usually just trot each side for 5 minutes then canter each side for 2.
Once she has gained some more, hill work really helps the top line! Just walking up hills on the trail really helped my mare's top line. We also did stretches like the carrot stretch on both sides, then out in front of her and then making her bow after every lunging session and after every ride. The top line is always the first to go and hardest to gain back unfortunately.
When I bestride her, I soar, I am a hawk. She trots the air, the earth sings when she touches it. She lends me the wings I lack