You have hit upon one of the huge frustrations of public easements.  Although you would think they fit the category of, "Highways, town ways and streets legally established,"   the fact is that towns have no obligation to provide any maintenance whatever on public easements, even though they are open to public use.  (See Town of Fayette v Manter, footnote 1: "... when a town discontinues a road and retains a "public easement," the public has an unfettered right of access over that road but the town has no maintenance responsibility.)  This is true regardless of how the public easement was created.

Roads in Windham come under the loop hole of 23 MRS 3105-A, whereby a town can vote to use its equipment on "private ways" (aka public easements) when it's in the best interest of the public for fire and police protection, but in Windham that is by a special agreement approved by the Legislature.  I don't believe even that agreement holds Windham's public easements to section 3651's "safe and convenient" standard.  I suppose it might be argued that by their agreement to provide winter maintenance, they have committed to keeping the roads safe and convenient during the winter, or at least to providing adequate snow removal.

If public easements DID come under section 3651, the remedy for non-compliance would be to proceed to section 3652.  I have seen this work rather well on town ways (not public easements, and certainly not on private roads.)  There is a procedure for it that must be followed carefully.  The first step is to notify the Town in writing of the defective condition of the road.  Describe the defect, point out that it makes the road "unsafe" and/or "inconvenient," and remind them of their duty under 23 MRS section 3651.  If it poses a special safety hazard, be sure to describe what could happen if the defect isn't fixed.  The letter should be delivered to one of the town officials, not just to a clerk.  You want to get them to date it and sign it to prove when they received it, and give you a copy of it with the signature and date.  Then you wait five days.  If they have taken no action, then you get three or more people to sign a petition to the County Commissioners, including a copy of your letter to the Town, and citing 23 MRS section 3652.  (Read through the several statutes in that series to get familiar with what happens after that.)  

I have seen this process used with success when there was a defect in the ENTRANCE to a public easement, or even a private road, so long as the defect is actually within the bounds of the town way.  (Look for stone wall boundaries, bearing in mind that town ways were usually laid out at three rods wide - 49 1/2 feet - and county ways at four rods wide - 66 feet.)  I know of one road association on a private road that had complained repeatedly last summer about a large pot hole that extended along the edge of the pavement of the town way where the private road joined it.  Each time they complained, they were told the town would fix it when they got to it.  But they never got to it.  The road association finally sent them a letter citing section 3651, and pointing out that in an intersection with notoriously poor visibility, a person swerving onto the wrong side of the road to avoid the pot hole could have a head-on collision with a vehicle trying to enter the road from the town way.  Amazingly, the town "got to it" within 24 hours!  

The other application I saw was when a town way was re-worked, cutting a corner and raising the grade by six or seven feet.   In the process, the bed of the town way was moved far to the inside of the 66 foot former county way right of way layout.  That left the entry to an adjoining public easement well within the right of way of the town way, and so steep it interfered with visibility.  (I almost drove into the side of a pickup truck as I exited the public easement one day, as I literally could not see it over the upturned nose of my car!)  The Town refused responsibility.  We appealed to the County, who came out and viewed the situation.  The Commissioners agreed that it was not safe, that the defect was well within the bounds of the town way, and that the Town had created the hazard by raising the grade of the town way without providing a proper entry to the public easement.  They ordered the Town to fix it.  The remedy involved raising a good stretch of the public easement in order to provide a reasonable grade with a  level landing spot where vehicles can stop before entering the traveled portion of the town way.

So there is a very limited possible application of section 3651 to public easements and even perhaps to private roads right at the entry, but beyond the bounds of the town way unfortunately I'm afraid it does not apply.  Again, you could try arguing the point when it comes to an agreement to provide winter maintenance - but I wonder if that would risk making the Town re-think its agreement.  You can read the statutes for yourself here:  https://legislature.maine.gov/legis/statutes/23/title23sec3651.html   

Use the arrow at the top right of that page to scroll through sections 3652 - 3655.