AI
The AI rarely blocks, but human opposition can be far less forgiving. Once you move out of the beginner circuit, start to think about how others will perceive your intentions, and don't make the lucrative trips too obvious.
Extra tickets
Taking on extra tickets when an opponent is below eight trains often leads to lost points and a wasted turn. If it is getting to crunch time, start to think about a few last-minute disruptions to others' lines.