Accessory Decoder outlet current

[FinnerZ]

I am planning to use the Hornby Accessory Decoder to controll a level crossing I am making, and I was wondering about the current supplied by the outlets. My decoders are inaccessable at present so I cannot conduct tests myself. Firstly, I was wondering if the current supplied by the outlets is DC. If so, does the polarity of the common outlet switch depending on which of the other outlets is being used (ie negative when the '+' outlet is in use and positive when the '-' outlet is in use). I am sorry if this is confusing - I can't think of a better way of expressing it without a diagram.