What constitutes an 'existing iPad Planet Auran account'?
One that was created prior to the fix being implemented.
I am wondering whether it is for people who own another Trainz product with a serial number.
Those people have never had a problem. The problem with downloading is specific to iPad-only users.
And does one have to purchase one of these tickets to download that I have noticed reference to?
No.
There is soon going to be an unmanageable list of mediocre experiments there that people have to wade through.
We'll have to wait and see

kind regards,
chris