You'll clear everything off the board except your best creature, which will enable an immediate attack and leave you in a winning position
The rate isn't bad for what you get, either, and the Great Train Heist will be useful at every stage of a game, whether you're using it for an early mana advantage, as a combat trick (offensively or defensively), or as a source of extra damage to close out a game
This is a great way to discard a Stadium you have in your hand
it can help bring out "Gravekeeper's Spy" or "Gravekeeper's Guard" and use their Flip Effects, allowing "Spy's" effect to be used to Special Summon high-level Gravekeeper monsters without tribute
When people think of Yu-Gi-Oh!, they think of Dark Magician - and in the early days of the card game before Synchro or Link monsters, a game-winning strategy could be as simple as copying your favourite anime character