I don't necessarily support a system of policy-making by referendum. I wouldn't necessarily want there to be a referendum on nationalising the railways. If the govt wants to do it, they should go ahead and try to do it. If this system were extended to include specific policies such as nationalising stuff, the system itself would need to be changed (to something more like what there is in Switzerland), making your question rather meaningless.
But as a general principle, I see no reason not to oppose and continue to oppose things voted for by referendum. Plenty of objectionable stuff has been voted through in Switzerland that I would most certainly oppose if I lived there. On balance, I don't think the Swiss system, where a question like nationalising railways might be put to a referendum, is a well-functioning one.