If it's any interest, the idea I mentioned was that due to weird and complicated reasons, a light railway I've created had an agreement with the Great Eastern Railway, which gave the light railway limited running rights on GER metals, between King's Lynn and London; and that similarly, the light railway has need to run a (comparatively short) regular boat train from a harbour it services, either to King's Lynn, or indeed all the way to London; and that post-Grouping, the arrangement is still in effect with the LNER.