Wiktionary
n. A ticket giving a non-traveller access to a railway station platform.
Wikipedia
A platform ticket is a type of rail ticket issued by some railway systems, permitting the bearer to access the platforms of a railway station, but not to board and use any train services. It allows people to walk with their friends and loved ones all the way to the passenger car at stations where the general public is not admitted to platforms. Railfans can also purchase platform tickets and enjoy their trainspotting hobbies. They vary in type: some may only allow limited access and a sharply limited time of usage, while others may have totally free access to enter the platform area. During peak usage hours or rush hours, the platforms may only be available for passengers who intend to travel.
Usage examples of "platform ticket".
The police and the Witch, I said to myself, probably don't feel like wasting money on a platform ticket, they will accost you and arrest you on the other side of the gate.
Standing beneath the announcement board, impatiently watching his men create the wrong kind of disturbance, then turning to the platform ticket machine and banging it because it appeared jammed or empty.