A few years ago, when FCC were unable to run their services due to weather conditions (snow) for an extended period, they gave affected season ticket holders 10 free tickets. These had a scratch-off date, rather like a lottery scratch card (with J, F, M, A, etc and 1-31 in numbers), a space to write origin/destination and a sticky-back plastic sealing cover. The plastic sealing and the scratch-off nature of the date selection rendered these single use. I thought these were pretty effective and would represent FCC's future carnet offering with out requiring any of this silly pen business - quite why they were never implemented in this format is a mystery to me!