A redcap is a train station porter, a worker who helps passengers carry their luggage. If you're traveling to Chicago by train and taking your stand up bass with you, you'll need the assistance of a redcap.
Redcap is an American nickname that's been used since the early 20th century, when porters actually wore red caps. Some people also use the words bearer or carrier — or sometimes baggage handler. This last term is more common at airports (along with the newer skycap), while porter and redcap are exclusive to railroad stations. British military police are also referred to as redcaps.