The train was being operated by CSX, but those cars are decidedly not owned or leased by CSX. Also, one news report I read said there was no track damage, so it doesn't sound track-caused, but it could have been (e.g., wide gauge).

As for repairs, they will likely have to be made by a contractor, which means someone is going to have to hire that contractor. That should be the entity who is contractually responsible (I would be shocked to learn these is no contract governing the existence of the walkway.) for the cost of repairs. The ultimate cost will almost certainly have a railroad labor component due to federal regs. governing working around railroad rights of way.