Business class sells out because it's a fairly cheap upgrade (starting at $20). The seats are 2 x 1 and have more a bit more seat pitch than, say, domestic first class, as well as a leg rest that extends.

You can sit anywhere on the train. Luggage goes overhead, or in racks at the end of the train. Overhead space is perhaps a bit more accommodating than a plane because it's not bin that closes.

I'd think you'd have to get tickets re-issued if you miss your train. Yes, they do sell out, but that tends to be mostly on peak travel days/holidays.

Tickets are fully refundable if not printed out, so you can always book a later train as a backup plan.