This device allows MetroCard holders to check their balance
and/or expiration date without affecting the data stored on the card. A
receptacle is provided on the side of the reader to deposit expired or empty
cards (most riders tend not bother to re-fill* cards they already have).
Uusually one of these will be located near the token booth, against a wall or a support column.
*("Pay per ride cards can be re-filled and re-used for about a year after they're first used. Unlimited-ride MetroCards expire for good when the number of days encoded into them have elapsed after first use)