As reported by the website China.org.cn the Beijing subway is allowing passengers to travel for free in exchange for waste plastic bottles. This initiative enables citizens to contribute to environmental protection and, at the same time, to save money.
Four "reverse vending" machines were installed on subway trains (for now only on line 10 trains) to collect plastic containers. Passengers receive between 5 and 15 dollar cents for every bottle. This means that with 15 bottles you can move freely on one of eight lines and 105 subway stations of  the Chinese capital.
The bottles are collected automatically and subsequently sent to a recycling facility. The service is still in the test stage, but should be extended to all Beijing subway stations.
The authorities are considering the possibility to use this service even on buses and other means of transport.