The fourth person to sit on a seat
built to seat three people. This term is used to refer to the unlucky person who gets to seat half their ass when traveling on a local train
's term for its railroad-based public transportation
. It is the commuting equivalent of a third wheel
Mumbai's local trains serve areas that lie as far as 60 km away from the city center. They are filled woefully far beyond capacity, even during non-rush hours. The sheer number of commuters on these local trains at all hours means that you can pretty much count on not having a seat when you travel. When you do get a chance to sit, you frequently end up as the fourth seat.
The fourth seat is an ignominious position indeed. You have to constantly push the seated half of your ass against the three others that rightfully belong on that seat and feel bad for doing so. At the same time, a nagging doubt gnaws on the corner of your soul wondering if you really are better off than all your standing co-commuters for having put in all this effort. Homophobes, who are particularly squeamish about close contact with a MOTSS, make for worse fourth seats, because Mumbai's local trains have sex-segregated carriages.
Then again, personal space is not nearly as big an issue in India as it is in the United States or elsewhere in the West.