The "Stranger Things" Season 4 teaser shows several prisoners working on a railroad which according to a theory, could be the Russians' way of locating The Upside Down.
A failed attempt to breach The Upside Down in Season 3 could have led the Russian soldiers to try to find another way to locate it. Fans may remember that they managed to crack a passage into the dark place with the help of a machine that drilled a hole into a rocky wall underneath Star Court mall.
However, Joyce, Chief Hopper, and Murray found a way inside the Russians' secret lab and decided to blow up the machine. The explosion led everyone to believe that Hopper is dead. Thankfully, he is alive, as revealed by the "Stranger Things" Season 4 teaser.
On a sad note, he is in Russia, toiling away on a railroad along with other prisoners of the Russian soldiers. The teaser made it a point to focus not just on Hopper's clean-shaven look but on his task at hand. It remains a mystery why he and the other captives are building a railroad through the Siberian wilderness.
According to Screenrant, if not to transport heavy freight, then the railroad could be another path towards The Upside Down. Perhaps the Russians are at it again and are trying to find another way inside the dark, mysterious place.
The publication pointed out the relevance of the railroad, given how The Upside Down has been discovered in the past seasons. In Season 1, Dr. Martin Brenner somehow managed to open it thanks to Eleven and her powers. They found its through an opening on a wall at secret Hawkins Lab. Nancy and Jonathan also found a way inside through an opening in a tree.
Then in Season 2, Dustin and Steve found a way inside The Upside Down through a hole in the ground. The gate/s to The Upside Down normally appear where there are weak points between its realm and the human world. Perhaps the Russians have found another gate and are trying to gain access to The Upside Down again.
This being said, The Upside Down likely becomes a major location in "Stranger Things" Season 4. The showrunners, The Duffer Brothers, teased that the installment will get down to the bottom of things and explore the origin of the dark place. The title tease basically also confirms that the story will revisit The Upside Down.