April's Cash Card is her ticket to ride - as soon as she hands her timesheet in to Stanley at the Fringe Cafe, she can get herself scanned at the subway entrance and start traveling around Newport. The Cash Card resembles a simple credit card that can have money paid straight onto it, probably using a reader, and is used to buy items in Stark.
It's likely that the card is linked directly to April's savings. At the start of TLJ, she has only $1 and her wages are about $375 (minus the $15 for her rail ticket), but she's able to buy a completely new outfit from her favourite - and incredibly expensive - store in order to head up to the MTI Building.
Automatically obtained at the beginning of the game from April's room in the Border House.