Waterslide Transit Systems

Oh but the exit gate does care!... The entry gate will read the card and the exit gate will read where you left the system and charge you accordingly.

So ...

What you are saying is that there is a contactless transaction on the way in and another contactless transaction on the way out.  That'll be two contactless transactions, both of which must have a monetary value of more than 0p.  Hmmm!  One low value ride split between two lower value contactless transactions.  I can see the business case faltering a bit here. 

Two questions for you?  First; on the way in, if you are not charging the full fare, how do you know the punter has enough money to pay on the way out?  Second; on the way out, how does the gate know where the punter got on?

And even if the exit gate does know where the punter got on, and can charge appropriately, what happens if the punter has no funds?

Next.

I suppose you could stop them, not let them out of the gate, call them on their mobile and make them top up their account over the phone. If you have their number and there are probably a few other ifs in there somewhere, like if they have the money in their account.

Of course you could ditch the card and just use the phone without all the swipes and save all that infrastructure and confusion.

I'm sure the travelers would appreciate the lower fares or better service that could be provided with the money saved, unless of course they also owned shares in BarclayCard or swipe reader manufacturers.

I see plenty of room for improvement.

Assuming you are referring to real-life, the way this is normally done is quite simple. The system debits the maximum cash-fare upon entry (on the tube, this is £4 I think), and refund the difference upon exit. There is, of course, the risk that the customer's balance is insufficient to cover the maximum cash-fare, even if they only want to use the system for a short journey. This is a risk that Transport for London, for example, is willing to take, and many a traveller have had to top up their Oyster cards when faced with this situation.

How the waterslide exit-gate is implemented is anyone's guess...

Certainly that is what happens with the Oyster Card - debit on the in-gate and credit the difference on the out-gate - but then the balance is essentially held on the card, so credits (or refunds) are instant.  Now, let's look at a standard MasterCard / Visa transaction - the debit is made at the point of entry, for £4 if you like, and then at the point of exit a refund of £2.50 is generated.  However, the refund in this case is not in real time, and is submitted to the card issuer via the settlement process, which means that it could take up to 3-4 days to appear on the account.  That means that for me to complete 4 trips a day for 3 days, I would need an overall availabler balance of £48, to enable me to enjoy £18 worth of waterslide.

This is one of those questions where the devil is certainly in the detail.  The answer is obvious until you lift up the hood and have a look!