UK’s Cold War Train from West Germany to West Berlin
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Peter Hebblethwaite's book "in the vatican" discusses the railway line which runs from Italy into the papal state (singular, history moved on from the plurality of states) and can deliver goods under bond into that tax domain. I think he rode it out once, but it's a supervised crossing and you really aren't meant to ride it in. I don't think anyone spies on that run, but it is a crossing of somewhat symbolic value but for accounting and "what is a state anyway" purposes has to be acknowledged. I guess if the train breaks down inside the vatican shunting yard the guy from Italian Railways who goes in to fix it can charge out-of-state costs. (I believe its basically a shunter which pushes 4 trucks in, and pulls them out. It might not even cross over the border, although the trucks do)