I traveled to Xining, and the food bag I took with me slowly bulged on the train. I don't know why.

If the food bag you take with you on the train bulges slowly, it may be due to the following reasons:

1. Air pressure change: When you arrive in Xining from a far place, as you get closer to Xining at a higher altitude, the air pressure gradually decreases, and the air pressure in the bag will change accordingly. In this case, the food bag may swell.

2. temperature change: temperature change may also cause food bags to swell. For example, if you go from a hot place to a cold place on a train, the gas in the bag may cool and expand.

3. chemical reaction: some foods may react with bags or other articles to generate gas, which will cause the bags to expand. For example, if the food in the bag reacts with some ingredients, it may produce gases such as carbon dioxide, which may cause the bag to expand.

if the food bag is swollen, it is recommended not to eat the food in it, because it may be unsafe. If you have any doubts about food safety, it is recommended to consult your local doctor or food safety agency.