I'd love to see how the TFL and Oyster network for London Underground is architected.

They have approximately 3 million journeys a day taking place, using RFID cards to gain entry in and out of the system. These cards are paid for either by monthly billing or as a cash top up from thousands of machines located in local shops and stations which can be used instantly. There is also an integrated website where you can pay for your travel pass, before swiping your card at the nominated station to 'collect' it. As well as that there is the route planning and real-time service information being delivered to the thousands of mobile internet users checking the best way home on their smartphone apps.

I don't know how they do it, but if SQL Server handled any part of all these real-time transactions and subsequent analysis this it would be impressive.