"Betas" is a function Steam provides so that players can play, well... beta versions of a game. In practice, it's also a way to play previous versions of the game.
Since TrackMaster is in early access, the term beta is not 100% correct, but I've just enabled it, allowing players to play previous versions. Right now, versions 0.8, 0.9 and 0.10 are available.
To access it, just RIGHT CLICK the game name on your library, then PROPERTIES, and then BETAS. From there, you can choose previous versions. I'll be using this feature from now on, so that you can always go back to a previous version if something is not working.
I've done this because right now, version 0.11 is not working on Linux.
I will soon release a new version of the game to fix the problem, and with this, I will anticipate some features of the next update.
Thanks for user berarma for pointing me to the issue on the forums.