You can get loads of points without paying interest with the card. I use my Amtrak card for all my household expenses that are payable by credit card, and then pay the card off every month. It is money I'd be spending anyway, so paying it off in full every month is no burden, and I get AGR points for my routine expenses.
You don't pay interest if you don't carry a balance.
I was unable to build up useful quantities of AGR points before I got the card. I have redeemed several cross country sleeper trips since.