Just like the government in a Command economy, the conductor has the power to start and stop the subway train whenever he/she wants. The government has the power to speed up or slow down the economy when it chooses.
Your metro card is like a form of currency in a traditional economy, where African tribes barter for goods. So you use your metro card to barter for a certain number of rides.
Wait.. what was that noise?
Was that our train? Uh oh....
Well just like the mixed economy, we can choose to get another train but we can't control how long the train stays at the station.
Also, it looks like you're going to missing practice today.