I agree with you.Do you have a source for this statement or is it your opinion? As senior railcards don't require photos and family & friends railcards don't need photos, it means your statement doesn't make sense. It may be difficult for a few disabled people to get their photo taken but as other railcards don't need photos there is nothing to suggest that this is the reason why. Oh, and blue badges require photos and people seem able to get them.
The problem is that is implying a facial disfigurement is something that people would want to hide away. Far more positive for it to be regarded as something just as likely to be on a railcard photo as any other face.But you have in that one sentence hit the crux of it.
Why should a disabled person have to rely on someone else to take a picture of them?
If they live alone for example and/or cannot operate a smartphone it is a barrier to them obtaining the goods and services they are entitled to. Hence the reasonable adjustment of not having to have a photograph on the railcard.
Likewise if someone has a facial disfigurement which they may/may not from time to time mask with make-up etc, it is a barrier to obtaining the Railcard. The DVLA issue driving licences without a facial picture to those with a facial disfigurment, the Disabled Railcard just goes further by not requiring it for any eligible disabled person.
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