This, unfortunately, seems to be the thing that Jeremy Corbyn is either ignorant of or chooses to ignore. The ROSCO business model is that they'll buy the trains on long-term finance deals from the builders, that are paid for by the long-term leasing contracts they have with TOCs, so effectively new trains are pretty much built on the model of a buy-to-let landlord. ROSCOs have a vast amount of capital tied up in them, and there's absolutely no way any government could afford the billions it would cost to buy them out.