从北京到上海的火车票价格是多少？请以json格式输出。
user
```json
{
  "from": "北京",
  "to": "上海",
  "train_tickets": {
    "hard_seat": "约174.5元",
    "soft_seat": "约288.5元",
    "hard_sleeper": "约304.5元",
    "soft_sleeper": "约474.5元"
  }
}
```
assistant