The current standard rail weight is in the 56-60# range, which is consistent with the weight of the largest rail in common use on the original Maine Two Footers.
My recollection is that the main line of the original WW&F was laid with 35#. Some of the original 35# rail has survived and some amount of that is in use on the museum's mainline today. However, the volunteers found that dealing with 35# rail is a lot like dealing with VERY HEAVY spaghetti - it wiggles and wobbles all over the place when loose. For this reason the museum has elected to make 56-60# rail its new mainline standard. It is much easier to handle and makes it possible for the WW&F to host some of the heavier surviving M2F locos in the future, should that opportunity present itself.
Best Regards,
Glenn