The famous CSGO Overpass decoy exploit still works in 2020

Overpass' train can still be stopped by decoy grenades five years after Cloud9's Jordan "n0thing" Gilbert used the now-illegal trick against Natus Vincere. Counter-Strike: Global Offensive's decoy grenades are rarely used, but a bug from the game's early days still works to let attacking players speed up their B site hits.

The first prominent instance of this bug being used came during the ESL Pro League Season 1 grand finals in 2015. Cloud9 was trailing Fnatic 2-1 in a best-of-five series. On C9's T side, n0thing's B short molotov meant to stop the CT rush was blocked by the train two rounds in a row. Whether it affected the outcome of the rounds is still a matter of debate, but n0thing thought so.

Back then, the map's train would run randomly, even during pauses and freeze time. CSGO community figure 3kliksphilip even broke down the issue in an excellent analysis of how it might have led to C9's eventual loss to Fnatic on the map, pointing out that the train could start randomly at certain points during the round. In competitive CSGO where timing is key, the train's randomness added another thing to worry about for attackers.

Why does CSGO's Overpass decoy trick still work?

There's actually a way for players to stop the train entirely, and it's all thanks to the one grenade nobody uses.

As shown by n0thing's blocked grenades, Valve gave the train collision detection inside CSGO's engine and anything with collision stops when hit by something else that has that same property. While molotovs turn into flames, smokes into smoke, and HE grenades explode outright, decoys are the only grenades that don't despawn immediately. They continue to exist and emit sounds and sparks until eventually disappearing.

But until then, CSGO's decoy grenades have collision detection that can stop the Overpass train.

n0thing actually used the trick against Natus Vincere after Cloud9's loss to Fnatic, and players can still perform the exploit today with the grenade. 

After n0thing used the trick against Natus Vincere, The train was eventually tuned to spawn at predictable times during rounds thanks to complaints from pro players in July 2015.

It's a small detail, but the trade-off was clear. With the change, Valve effectively stopped players from using the train's loud rumble to mask executes. Players probably shouldn't attempt the trick in competitive matches. Even though it still works, it's unclear if Valve considers it a bannable offense. Overwatch may consider it one, so it's better to stay away.