I would doubt that anywhere near 30% of Amtrak riders each day are AGR members. Maybe for business commuters on the corridors, but for the system as a whole, I doubt it.
I concur with your assessment — I believe that the number of AGR members could be dramatically higher on the main corridors, including NEC and California service. That's why I include a ratio of corridor passengers to total passengers in my original estimate of 11,764 AGR travelers earning 170 points.
That number is about 33 percent of total estimated daily round-trip travelers, and I hope is a fair representation of commuters and potential AGR members.
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My trip was really fun although it included waking up at 4:15 a.m. so I could drive to Sacramento for a 6:40 a.m. San Joaquin. I'm glad I drove, I thought about taking the southbound Starlight but it pulled in a scant minute before the San Joaquin left.
I've got photos and I hope to work up a travelogue over the weekend because it was a lot of fun.