No password is required, you just need to opt in to the beta content on Steam. You can do so by right clicking on the game in your Steam client, going to properties and then the beta tab. Once there, just select the beta from the dropdown box to begin downloading. They're asking for feedback here if you find any issues. It doesn't have a SteamOS icon yet, since the Linux version is not fully released.
Seeing games like this get Linux support, really makes me happy! I absolutely love building and simulation management games like this. You can end up spending hours just watching everything, improving your buildings and it actually looks pretty good.
You will be tasked with creating the entire station, from the tracks to the cafés and bars to keep your passengers busy while they wait. It's not finished yet though, as it's currently in Early Access. What they have planned sounds awesome though including: weather events, disasters, more advanced AI and so on.