Hi Ted,
Over the past several years the SR&RL folks have relaid their entire mainline with rail in the "40# and up" category to conform with MNG's requirements. Their turntable has also been upgraded to handle a loco with a 6-to-7 ton axle loading, like #3. And finally, they rebuilt the bridge at "Sanders" to standards sufficient for carrying a 38-ton locomotive.
Even so, 35# would work fine for an 18 ton locomotive. The original Monson used it extensively, if not solely. The Kennebec Central, which also ran 18-ton Forneys, actually used 25# rail on their mainline, with no problems.
Best Regards,
Glenn