I would not characterize the Island Explorer an "extensive shuttle bus system". It's better than nothing - I remember when there was nothing! - it's used moderately well on the island, but its primary drawbacks are lack of frequency and lack of off-island reach. If you are going to Acadia and expecting to not use a car, you are still driving to Acadia, and staying on the island, probably in or very near Bar Harbor. If you're staying in Ellsworth, which is 20 miles from Bar Harbor never mind the rest of Acadia National Park, or anywhere else in the area off island, or even if you are staying on the "quiet side" of the island west of Somes Sound, you are driving, period. The closest that the Island Explorer gets to Ellsworth at present is the Trenton Airport. Yes, there is also Downeast Transportation bus service that connects Bar Harbor to Ellsworth and even Bangor, but it is very infrequent as well.
Access to Acadia National Park, by any land or air-based means, is not by "roads", it is by *a* road. One single road, Route 3, which is known as High Street where it runs right smack dab through the middle of the central business district in Ellsworth, and which also carries Route 1 through town. Congestion already is vastly disproportionate to the size of the town, since it's the only way to "get theyah from heah" by road. The historical location of the railroad station in Ellsworth is also along High Street, and no matter where you might site a future railroad station, to get to Bar Harbor you have to get onto High Street. There is literally no other route. You can only avoid passing through Ellsworth by flying into Trenton Airport, but since that is also along Route 3, you're just dropped into the mess of traffic further along. It is a matter of georgraphy as much as it is infrastructure.
Fact of the matter is, you could put everybody coming from anywhere via any conveyance you choose, trains, planes, or automobiles, and if you pack every one of them onto buses to go to Bar Harbor and Acadia (where would that enormous Park and Ride be?), Route 3 will STILL be a parking lot during the summer.
A rail option would be great, but it would be seasonal at best (I'm the only Flatlander I know who actually wants to go to Ellsworth in wintertime) and it would cost billions of dollars to not only bring the in-use portion of railroad to Bangor up to passenger standards, but worse, despite Downeast Scenic's care and effort, the nearly 40 years dead portion of the Calais Branch needs massive investment too (and like Portland, don't forget the Calais Branch is accessible from the Freight Main only by a reverse move at Brewer Junction).
I grew up in the Ellsworth area and still have family there. Of course I'd love to take a train there. But not at that cost. I'm not convinced Bangor is worth it either, but rail to Ellsworth to get to Acadia? NOPE.