It's something CX can give you if your gate is far away, and if you are checking-in at the F counter (do J counter give them out, I don't know.) However, it's not something free on CX, I'm sure the airport charges CX per ride. In the past, I usually asked really nicely that since Wing is down, can they please give me a voucher, just one, so I can take the train to Pier, but ride it back to gates 1-4 (where JFK planes are usually gated.) This time, they said no, no complains, very grateful.