Highspeed services elsewhere in Kent will be cut. Normally, 2tph would run to Faversham... instead, 1tph will run to Faversham. Also, 1tph would normally run to Margate and 1tph would normally run to Dover Priory... instead, these will both be terminated at Ashford. This frees up a couple of trains. There are also often a few trains stabled each day, so these will just be being used.
Also, the trip from Ebbsfleet Int. to St Pancras Int. is only 20 minutes, so not many trains are needed to run a high frequency service.
Wishful thinking that FGW would be able to do the same, haha!
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Nope, Oyster cards can not be used on HS1. 'Games Travelcards'
can be used, as previously mentioned.