Not sure if that's the proper term but that's what I heard chili_n_such call it. I looked everywhere for a tutorial but I couldn't find anything.

After ~1.5 seconds has passed, save load 9 times (depending on how powerful your computer is) without letting the timer progress and the train behind you should be de-synced into the train you are riding.

Check my tutorial video:

Keep in mind timing and amount of saveloads can vary depending on your hardware. I'd say try 9 saveloads, like Taggart just said.