When you form a chat-like conversation over the email or resend email messages by replying again and again to the same message and you have a long train of emails or messages attached. Also known as a Email thread or Email Train. This can also occur if you are texting back and forth and have a long message train.
Wow.. look at this Message train we have going. It goes back 10 messages. or Dude! I have a message train of about 50 on my iphone!