This *must* have been discussed here before, but I've done some searching
on Google Groups and can't find the answer. Why doesn't TfL sell a one day
travelcard on Oyster, to be used by those who need to travel on National
Rail? It seems any of the arguments involving poor Oyster facilities on NR
would also apply to period travelcards. The only thing I can think of is
that it might confuse customers who can't figure out the difference with
capping.

It certainly has been discussed but I am not sure that a definitive answer
has been provided or even a consensus of opinion beyond a suspicion that it
probably isn't worth the effort or the hassle that any extra confusion may
cause, and that is certainly my suspicion.

It does seem like it ought to be possible to implement a one day travelcard
as a one day period travelcard though perhaps the system doesn't have the
concept of an off-peak only period travelcard. If it could be implemented
any such cards presumably would have to be located in the two (?) period
travelcard slots on the Oyster card and that might be considered rather
inflexible and a source of confusion. I know I have bought two or three one
day travelcards in advance in my more organised moments and there have been
occasions when I've wanted a couple of one day cards when I've already got a
period travelcard and that wouldn't be possible.

G.