The youth, who cannot be named for legal reasons, carried out his first robbery at Wembley Park station on April 24 this year and a second on a Metropolitan line train between Rayners Lane and Finchley Road two days later.
Blackfriars Crown Court heard his first victim, a 17-year-old from Wembley was forced to hand over his phone and body warmer.
His second victim, a 17-year-old boy from Uxbridge refused to hand over his belongings even though the robber threatened to stab him.
However he was frightened into giving him his watch, mobile phone and £20 when his attacker started counting down from 10.
The youth was snared when a British Transport Police officer patrolling Rayners Lane station spotted him as the teenager wanted in connection with the two robberies.
He tried to flee the scene but was caught and arrested.
He pleading guilty at an earlier hearing to two counts of robbery and two counts of possessing a knife blade and was sentenced last week.
Detective Sergeant Thomas Madden, the investigating officer, said: “This was a terrifying ordeal for both victims, who feared they would be stabbed if they didn’t hand over their belongings.
“Carrying a weapon on the railway network is completely unacceptable and BTP takes it very seriously. Anyone caught with a weapon will be arrested and prosecuted.
“While violent crime on the Tube is fortunately very rare, any incident like this is unacceptable and we will do everything in our power to bring offenders to justice.”