From: johnl@iecc.com   
      
   >>>We're talking about transit fares. All that has to be done in real   
   >>>time is validate the fare media. The rest of it doesn't have to happen   
   >>>immediately.   
   >   
   >>Transit fares can be pretty complicated.   
   >   
   >You gave an example in which you were billed in arrears.   
      
   The credit card pseudo-Oyster is billed in arrears, but I'd guess that   
   they get approval for the price of a travecard (the most you can spend   
   in one day) the first time you tap in.  Or they may figure that the   
   risk that the charge will bounce is worth it for the value of not   
   having to produce and manage cards for a million casual users.   
      
   The Caltrain examples are prepaid.  There are several chunks of   
   prepaid value, in this case an eight-ride ticket and cash, and it   
   needs to debit the right one when you tap out.   
      
   Smartcard systems go to considerable effort to allow offline   
   operation, with info stored on the card and clever crypto to   
   authenticate cards and terminals to each other.  If it'd really be   
   easy to do it all offline later, why do it the hard way?   
      
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