Back Up The Train 

used to say, "hold up" or "wait a minute" or "thats not true" or to express disbelief.

1. A loud girl, Joanna is talking to a boy, Liam for the first time:
Liam: you seem really quiet Joanna
Joanna: Back up the Train! I never shut up!

2. Joanna is talking to her friend Lisa later that day:
Joanna: so I talked to Liam today!
Lisa: BACK UP THE TRAIN YOU LUCKY DUCK!

3. Lisa is talking to Rebecca later
Lisa: Rebecca you're kinda annoy-
Rebecca: Backup the train & don't let that thought out of the station!

Back Up The Train by ewitsmelala January 28, 2010

backup the train 

An expression used to stop something or slow it down. If someone is talking fast, backup the train will mean for them to repeat what they said. Can be used with hold the phone

Kids in a classroom going crazy, someone swears
Teacher: Hold the phone, backup the train! Who said that?

backup the train by Don C. Hawantme June 20, 2007