TKD said:

I live in south London, and can't use Oyster for my season ticket to
Croydon, because it's not valid on trains.

You are misinformed. Any season ticket sold on Oyster Card can be
used on the NR trains for all the zones it is valid.

My season ticket isn't valid in ANY zones. It's valid only between my
local station and Croydon.
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