David of Broadway wrote:

Most of the NR system is outside the zones, and most NR stations -- the
ones outside the London region -- don't have and will presumably never
have have Oyster pads. Oyster pads on the trains would be useful for
people traveling between points outside the London region and points
inside the London region.

How? Given the existence of combined day return/Travelcard tickets,
combined train/tube tickets, and Boundary Zone tickets, I can't think
of many journeys that would really benefit from this.

U