Moment to moment different trains of thought are passing through the mind. With mindfulness, we choose which thoughts to entertain and which to abandon as if we're standing on the platform of the station deciding on which train to board. In order to arrive at the desired destination, we invest careful consideration into choosing which direction we want to travel. The question arises, do we want to board the train bound for suffering, or would we rather reach its opposite?
From our prior experiences we begin to discern which direction the train is heading before we blindly jump on board. We begin to question "where will this thought lead?" and "is this the direction I want to go?"
Once we're on board, we have to realize that the thoughts we add are actually laying the track upon which the train continues to travel. In other words, if we're caught in a negative loop and we refuse to continue thinking similar thoughts, we derail the train.
And as for when we're thinking beautiful thoughts of loving kindness?...keep laying the track.
In this way we become the conductor and engineer of our own minds.