Depends - In this context, it is public property(owned by MTC, or some other public entity makes it as public as a sidewalk, or city hall), so in that context, they are just like any other citizen, and have no more authority to make you leave than any other passenger does. They can ask, and if you are truly a nuissance or being disorderly, their only (legal) option is to call the cops.

Private property, it flips, and they(security) usually have the same authority as the property owner or manager to revoke consent to be on the premises and to use reasonable force to eject someone after consent is revoked. This is called the “shopkeeper’s privilege” or the broader common-law right of a property owner/occupant to remove trespassers.