This is the key to solve this kind of application problems, and the distance must be clearly stated. If the train passes a bridge (or tunnel), the distance traveled by the train from the front bridge (tunnel) to the back bridge (tunnel) is equal to "the length of the bridge plus the length of the train". For example, two trains face each other and want to leave from the front. The two trains are the sum of the body lengths of the two trains. So as long as the distance is clear, the following solution method is basically the same as the solution method of trip problem. For example, the so-called "crossing the bridge" in your question means that the distance traveled by the train is equal to

The length of the bridge plus the length of the body. Then, according to the relationship of the trip problem: time = distance-speed, the time required for the whole car to pass can be calculated. (3 15+ 1035)30 = 45 seconds