The train I went to did not have door knob how do I get in? Am I trespassing?

No, entering a train is not trespassing as long as you have a valid ticket or entered the platform legitimately, and that it is a passenger service. Trespassing means to enter an area without authorization. By buying a ticket (for example, the Metro-North) or paying to enter the station (for example, the London Underground), you are given permission to be there. Now for the main question, if the door does not have any button or knob, you should assume that the door is a power-operated sliding door or door that the driver/guard operates. These doors should open once the train has stopped (unless the train is not scheduled to stop there) unless the door is defective.