Solution: (1) Let the length of the train be x meters, the distance from the front of the train passing under the light to the rear of the train passing under the light be x meters, and the average speed of the train during this time is x/ 10 meter/hour. (2) Let the length of the train be x meters, and the distance from the front of the train entering the tunnel to the rear of the train leaving the tunnel is 300+x meters. The average train speed during this period is (300+X)/20m/h (3) The average train speed has not changed in this problem. (4) According to the meaning of the question, we can list x/ 10=(300+x)/20, and solve this equation to get x=300.