WAGN
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Between Finsbury Park and Hadley Wood or Crews Hill.

Why not?

Because there are no Oyster readers.

That's not the only reason. For example, there are Oyster readers at
Willesden Junction and at Gunnersbury, but even so, pre-pay isn't
valid on Silverlink between the two (despite the fact that TfL zonal
tickets are valid on the NLL, and ticket inspectors on the trains need
hand-held Oyster readers anyway because travelcard-on-Oyster is
valid).