Firstly, lucky you - on my train they charge you silly money with a minimum purchase of a quarter of an hour.
How you access it depends on what you're using - my phone has a 'browse for wireless LANs' function. If you're using a laptop, different versions of Windows do it in different ways but try right-clicking the wi-fi icon in the notification area of your task bar and there should be an option to view available wireless connections. It should then just be a case of picking the right one and following the wizard.
Nelviticus