An email train refers to the long trail of previous messages that is formed when the people in the email reply without deleting the previous replies. In some cases, an email train can be useful to track the progression of a conversation, much like discussion boards do. However, the creation of email trains is usually frowned upon as poor email etiquette.
Techopedia explains Email Train
In many cases, email trains are created by laziness as people “reply all” on emails with more than one recipient and then others continue the trend. Instead of adding to the email train, a participant can delete all the quoted text on his or her reply or just leave the text that is relative to the discussion rather than keeping it all. While email trains have never been a make or break issue for email users, web-based email programs have tried various methods to eliminate email trains by organizing serial emails into conversations, automatically trimming quoted content and so on.