I don't think the 35 pound rail on the Harrison branch was ever replaced--- which is probably why it was abandoned, after #8 rolled over. 3 has much lighter axle loadings though than 7 or 8 however. Your point is legitimate however, I know that when they started they had some 16 pound rail. I think that's been resolved for a long time though. They also had a bridge that was light. I am sure there won't be any problems with the "physical plant" at the SR&RL museum. I remember people in the museum say a while ago that the issues that had prevented 3 possibly travelling to the SR&RL had been resolved.
It'd be a bit of a stretch to say that the engine ran on the B&SR... like I said before we weren't within 5 miles of the original B&SR grade that was right on the waterfront. We did bring steam to Harrison for the first time in 75 years however.
Regards,
Trevor H.