Perhaps more relevant to this discussion than you might think is that EZPass doesn't work outside of the EZPass district - which, to be fair, is most of the Northeast and some of the Midwest. However, you're in for a rude surprise if you try to use your EZPass in Florida.
In theory, EZPass and Sunpass (as well as EZPass and all of the west coast agencies) are going to be interoperable by 2016, but I'm not holding my breath.
You'd be surprised how interoperable it's gotten already. 10 years ago it was only the 3 founding EZPass states: NY, NJ, and PA. Now DE, IL, IN, MA, ME, MD, NC, NH, OH, RI, VA, and WV have joined ,as well as Ontario for the Canadian side of the Peace Bridge to NY. Usable to pay for parking at all Port Authority airports in NY/NJ, Pittsburgh Int'l, and at Westwood/128 station (hopefully a sign of some future Charlie and EZPass interoperability at T garages). In VA their next-gen transponders can tell when you're traveling in a regular pay lane or in the free HOV lanes. And NY also did a short-lived pilot program for paying at rest stop McDonald's drive-thru's.
The only stragglers in the member states are several bridges run by independent authorities that opted out of the program, the OH/KY border, and the other 6 NY/Ontario border crossing bridges (which are under negotiation). I don't know what the warring camps are nationwide for interoperability, but EZPass is well on its way to ruling all east-of-Mississippi unless Florida exerts pushback. That's not bad. Even if the integration stalemated at, like, 3 different regions that is a hell of a lot of progress in a short decade. It's to the point where USDOT could get involved, especially if they start relaxing the tolling rules for Interstates.
It's just too damn bad that you can't actually use Charlie on the Providence Line, nor can you on any other commuter rail service, and that implementing functionality to use Charlie on RIDOT trains would add additional layers of complexity rather than simplifying things.
I don't know what exactly the hangup is with commuter rail, but it apparently is legitimately more complicated than anticipated. It's no big deal if it takes half the decade to get it right, but the T needs to be roundly criticized for throwing in the towel on even trying and thinking those smartphone tix are going to cover all the potential customer base. I don't have even have a data plan with my phone. If I want to catch the Fitchburg Line out of Porter like I did a couple times last year I don't relish the thought of getting jacked for a higher fare if I don't go downtown to pre-buy.