Not to say ATV's wont be an issue, we are extremely fortunate to have some very caring and considerate neighbors.
At minimum the ROW and especially the bridge + approach trestles should be posted for No Trespassing since not everyone is as considerate as our good neighbors and friends. We should be posting all crossings with such discrete signs -

Legally the signs might have to be posted by the W&Q since that is the entity that the Museum is leveraging.
U.S. Code › Title 49 › Subtitle V › Part A › Chapter 201 › Subchapter II › § 20151
- Railroad trespassing, vandalism, and highway-rail grade crossing warning sign violation prevention strategyhttps://www.law.cornell.edu/uscode/text/49/20151 <- Thanks go out to Cornell Law School!
(c)Model Legislation.—
(1) Within 18 months after November 2, 1994, the Secretary, after consultation with State and local governments and railroad carriers, shall develop and make available to State and local governments model State legislation providing for—
(A) civil or criminal penalties, or both, for vandalism of railroad equipment or property which could affect the safety of the public or of railroad employees; and
(B) civil or criminal penalties, or both, for trespassing on a railroad owned or leased right-of-way.
I'll see if I can dig up the specific Maine legislation for RR ROW Trespass.
I'd urge the BoD to consider it as an agenda item.