The 42nd Street shuttle aka the "S" Train runs back and forth between Grand Central & Times Square. After a couple of weeks of back and forth running, the sets used on this line must return to the shops for servicing, inspections, wrap changes, etc. Because of the track layout, The sets on Tracks 1 & 3 must utilize the Tunnel that connects with the the Lexington Ave Line & the set on Track 4 must utilize the connection to the Seventh Ave Line. In this video you'll see the set from Track 3 (The only 4 car set) get swapped. This process takes roughly 30 mins (Taken 1/15/15) *You can watch the replacement set deadheading here: