I think a scan of the passport/birth cert - I haven't looked into it that closely as I thought it was a gross invasion of privacy for the sake of free bus travel.
do they have to?
when I applied for a ZIP card for my son last year it wanted me to fill out a online form with his and my details, take a photo and upload it for the card and give passport number as proof of ID, he doesn't have one, so the option for me was submit application without proof of ID, and get a deceleration letter I had to take to the post office with his birth certificate, the person at the post office made no copies or anything he just looked at the birth certificate and then signed and put a stamp on the letter, I then posted the signed deceleration letter.