I'm guessing besides GPS, these points are key for the T:
* Priority Ethernet Port for On-Board Systems
* VPN/802.11 Support for Data Transfer to HQ Database
Meaning these WAPs also let them connect any random thing they want on the train car back to MBTA HQ. Things like automated announcements, variable message signs, vehicle diagnostics, fare collection remote terminals, etc., could all be updating in real time. If I were the T, I'd think customer WiFi is just a bonus compared to that.
With regard to a pay-per-use model, I don't think that will fly. Most businesses that provide incidental WiFi don't charge for it. There's just too many free options out there to attract many users to a pay service. Why pay the T $10/month forever when I can get an
Aircard for $50 -- or better yet, convince my company to give me one for free?
I do think the "sponsorship" model would work for the T. I certainly wouldn't mind clicking through an ad to get internet access. That would also be easier for the T to administer than a bazillion individual accounts and payments.