Having heard of some of the States other "branchline" refurbishment requirements, I would guess they are costing a steel bridge with concrete / ballasted deck and broad walkways. All kinds of heavy erecting machinery and environmental impact to be mitigated. The foundation is probably "too old" so the creek will have to be dug out too. And someone suggested prevailing wage which sounds plausible. Don't know if this approaches $5 million but I doubt they consider a wood trestle "acceptable". It seems they would rather have a gold plated feeder system or nothing.