Use it however you want.
There isn't really a "right" way to use MTT. Some of us use it to keep a list of our model buses, some use it to track stuff for other games, and some just play it as a standalone thing. We've split the new version into two sides to keep it simple.
MTT (www)
This is the "do what you like" side. It's a total sandbox where you can draw your routes, list your fleet, and make your own timetables. There's no money and no way to "lose", it's just for building your world and showing it off.
Go to MTT →Play (play)
This is more of a game. We added an economy so you have to manage your bank balance, buy and sell buses with other people, and try to run a company without going bust.
Jump into the game →The best part
The two sides talk to each other. If you set your fleet up on the main site, it syncs over to the game. You can mess around with routes for free, and when you're ready to see if you can make a profit, your buses will be there waiting for you.