using the less expensive seats or accommodation in a train, ship, etc.乘坐（火车、轮船等的）二等舱◆to travel second class坐二等舱旅行 by the slower and cheaper form of mail通过第二类邮件投寄◆I never send important letters second class.我从来不用第二类邮件方式投寄重要信件。