In this dream, I dreamed of "getting on the train by myself". A train is a means of transportation. Its function is to let others go to other places. In reality, the significance of paper bag is a change and opportunity. You mentioned above that "you are about to face an internship", and you have to get in touch with new things and new environments, so this dream appeared.
"The wallet is in the bag", and money is indispensable to people's lives and affects people's daily lives. In the dream, "I don't know why I disappeared after I got on the train, but I couldn't find it for a long time" shows that you are in a state of panic, confused and nervous about the interns.
Although I lost my wallet, I still have a big bag and a mobile phone in my hand. This is what you have, that is, your accumulated knowledge and experience and your own conditions.
Dreams reflect your anxious state of mind, but you should clearly realize that what you have is a little. Don't panic. Compared with others, you are a newcomer and have a learning process. Others are immature and steady, that is, "money" that can't be found for the time being, and can be accumulated slowly after work. This is the process of your maturity.