The Sonic Train Glitch is a glitch in Super Smash Bros. for Nintendo 3DS.
Characters this Glitch is performable with
Stages this Glitch is performable on
How to perform
Wait for a gray train from the front of the train. When it hits, go into the little space and start running (as shown below) and Sonic will run through the wall. This also works with Captain Falcon.
However, due to a patch update, this no longer works.