Fascinating Stewart!
if you look at the way Alna is laid out (along with it's principal structures and industry) you can understand the reasoning behind all four stops.
Sheepscot was meant to serve the community that existed on each side of the Sheepscot River down east of the museum on Sheepcot Road. I've been told and seen some pictures of a mill that once stood by the reversing falls, so this may have been served by the Sheepcot stop as well. I'm not sure when a bridge was built at the location, but I think it must've been early.
Alna Center is located in a good position to serve both the meetinghouse and the school, both of which predated the railway. As has been said, there was once a road which went all the way across to the West Alna Road where Alna Center Station is, so it served other functions for the residents as well.
Alna would have served the part of Alna that is referred to as Puddledock, which you get to if you take the right turn at the fork by the Alna Store. There was a post office, store and church there among other things. Not sure when the bridge across the Sheepscot was built here either, but this would've given those traveling direct access to the station from town.
Lastly, Head Tide, which was the location of a mill, store, blackmith shop, school, and church. This stop was significant enough that it warranted a larger station and siding. Most of these buildings are still there, aside from the mill which burned and the blacksmith shop.
The teacher in me has taken note that each of these villages once had their own school. I believe all of the buildings still stand. Although none of theme see students anymore, the Alna Center school has been preserved by the town.
If you are interested in the history of Alna, I'd strongly recommend getting a copy of the film "Currents, Courses, and Streams, Alna, Maine: Histories of a Backwater Town". It is very well done and does a good job of expressing what life was like in Alna years ago. Plus, there is a small section on the railway and it's role in the community. I believe the film is available from the gift shop.
Steve