It's all about a single individual making a big difference.

The video of a fun-filled train journey posted on the YouTube has become an overnight sensation. A Mexican woman travelling on an underground train in Berlin starts laughing at something on her phone. The passengers around her too start smiling. In no time, all the other passengers join her, sending the entire carriage into a giggling spree.

A total of 1,800,060 people have already watched the three-minute footage which has been titled Lachen in der U-Bahn.

One of the comments on the YouTube post claimed: "i was there,the joke was: "why did hitler suicide? because he had n money for the gas bill!"