That's when an advertiser buys all the space on an entire train and plasters its own ads all over it. (thus hi-'jacking' the train with ads)
This one was for Lenor, a P&G fabric softener.
The cool thing is that they hung these shirts up (real shirts - it's Japan so no one steals or vandalizes them!) with 'sweat' dripping. On the sweat drop, and on a puddle sticker on the floor, the ads talk about preventing sweat smell on your clothes with Lenor.
Brilliant idea for the sweaty summer Tokyo rainy season.