THE MORNING commute on line four of the Chongqing subway is, as anywhere, full of workers rushing onto the train and going downtown. But there is a crucial difference. The workers are not destined for offices. They are farmers carrying large baskets laden with beans, tomatoes and more as they head to markets. It is a scene that illustrates one of China’s marvels: a sparkling subway that links the countryside to the city. Alas, it also illustrates one of China’s glaring gaps: pitiful pensions that leave many rural elderly with no choice but to toil.