Complex sentence: When the line was constructed several stations were left with a straight tunnel provision for potential future construction one of which was Gorkovskaya (now Tverskaya) between Mayakovskaya and Teatrlnaya was opened in 1979.
Split into simpler sentences: When the line was built several areas were left with a straight tunnel provision for potential future built in of new stations. One of which was Gorkovskaya (now Tverskaya) between Mayakovskaya and Teatralnaya, which was opened in 1979.