如何用代码实现这个回溯算法？

用Python编写一个简单的回溯算法来解决这个问题，我们首先需要创建一个函数来表示火车的状态，并在递归过程中生成所有可能的调度序列。这里是一个基本的例子：

def train_schedule(n, trains, current_schedule=[]):
    # Base case: if all trains have been scheduled, return the result
    if len(current_schedule) == n:
        print("Solution:", current_schedule)
        return
    
    # Try to add each train to the current schedule
    for i in range(1, n + 1):
        # Check if the train is already in the current schedule or if it can be added
        if i not in current_schedule and (i == 1 or trains[i - 1] == ''):
            # Add the train to the current schedule and move to the next one
            current_schedule.append(i)
            train_schedule(n, trains, current_schedule)  # Recursive call
            # Backtrack by removing the last added train
            current_schedule.pop()

# Test the function with the example of 4 trains
trains = ['', '', '', '']
train_schedule(4, trains)

在这个例子中，`trains`列表代表火车到达的先后顺序，空字符串''表示尚未处理的火车。`current_schedule`变量存储当前的调度状态。函数会递归地尝试将每个火车添加到调度中，然后在每个递归层级结束后回溯。