Bad manners not to cancel it, but if you had a reservation for (say) the 18:45 and decided to leave earlier, you could hop on the 17:45 and sit in the unreserved cars, no problem. If you know your itinerary, there's no reason why you can't make all the necessary reservations ...
a general purpose computer owned by the customer are: (i) portable purchasing device 110 runs a dedicated purchasing application with a friendly user interface that does not requires knowledge of using URL addresses or entering websites; (ii) the customer may not have a computer in the car. ...