How do cabcharges work? I sometimes get given them by companies that I have visited in the CBD, but sometimes it is just as easy to get the train back instead. Can you keep them and use them on a similar route another time instead, such as after going out for dinner in the city? Or do they have some kind of date/cost restriction on them?

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Edit: Sounds like the answer is that you can use them another time, but you shouldn't. Thanks all!