Definition of the Transit Transaction Model (TTM)
4.15 The TTM is a new Contactless payment model
established by TfL in collaboration with Visa, MasterCard and American Express. It has been designed to support operational constraints of transit environments and joins the worlds of transit and payment together. The TTM is now mandated on all card issuers worldwide by Mastercard and Amex and in Europe by Visa Europe. TfL’s implementation of the TTM is the first and so far the only example of this, which makes TfL’s intellectual property on this particularly valuable.
4.16 The need for a TTM arose as the way Contactless cards are used in the retail environment is not suitable for the transit environment due to two critical differences between retail and transit. First, the value of the payment is not known in transit at the time of using a Contactless card on rail; it only becomes known when a journey has been completed, with two or more touches of a Contactless card. Second,
the methods to control risk in the retail environment require the card to default to Chip and Pin from time to time, which cannot be supported in the high intensity transit environment.
http://content.tfl.gov.uk/aac-20141217-part-1-item12-contactless-payments.pdf