Fortnite's train, along with armored transport trucks, both contain vaults you can open with thermite. To gain access to an armored transport truck and its vault, you'll need to follow a truck's ...

Pick up the Sprite and return it to Bushranger, and with a successful trade, he will provide you with the vault key that you can use to open the vault in Nightshift Forest.

Fortnite Chapter 6 Season 1 is full of mysteries, including the secret Nightshift Forest vault and here is how to open it.