What automatically drains from the air reservoir?

The correct choice is water. In air brake systems used by trains, air reservoirs collect compressed air to be used for operating the brakes. However, as air is compressed and then released, moisture often condenses from the air due to the cooling that occurs during compression. This moisture builds up in the air reservoir over time, which can lead to issues such as freezing or corrosion within the braking system if not properly managed.

To prevent these problems, most air reservoirs are designed with a drain valve that allows water to escape automatically or be manually drained as necessary. This process is essential for maintaining the integrity and functionality of the braking system, ensuring safety and efficiency in train operations. Unlike the other options, which do not get drained from the reservoir, water is a byproduct of the air compression process that requires attention.