It’s really stupid of Princess Leia to read the book named Galactic Rebellion for Dummies in public. Take a look, the Stormtroopers had caught her in the subway car.
No doubt, it’s a surprise operation authorized by Darth Vader. apparently Princess Leia had already been traced. Those stormtrooper soldiers quickly found out her in such a confined space. Darth Vader was waiting for the fated meeting with Leia at the next stop.
Well, in fact, Imorov Everywhere directed the action happened in the subway car. Undoubtedly this successful action has achieved the purpose of entertaining people in public places, especially the over height Darth Vader and the funny book for Rebel Alliance.
By the way, there is evidence that Darth Vader was on to subway from the very beginning.