In versions of Trainz before MPS was added in place of the old multiplayer system, it was fairly easy to see games in progress. You would pick a multiplayer session and open it in Driver and a popup windows showed you the active games and whether they were public or private. You just clicked on a public games and the person running the game would let you join. I don't see the same functionally in TRS22 with just MPS but maybe I don't know where to look.
But the bigger problem is there just aren't that many people playing MPS at the moment. That has always been an issue. You might have better luck asking in the Multiplayer forums to see if someone running a session will let you join the next time they are running a game.