On 06/08/2016 10:10, tim... wrote:
There is always going to have to be a system of purchasing tickets by
walk ups - even if the actual "ticket" is dispensed on a one time use
smart card..

I was in Seoul last weekend and that's what they do there. The cost of
the journey includes a deposit of 500 won (approximately 35p) for the
ticket. There are machines outside the gateline that will accept the
card and return the deposit.