I got to try the new HvH metro link a couple of weeks back, en route from London to Hamburg. All very modern and swish, but a bit disappointing compared to the old days when there were "proper" railway carriages sitting there next to the boat to take you, direct, half way across Europe... Also, waiting a while to set off - on one of those old long-distance connections - was fine; but standing on a windy platform for half an hour (it was a weekend, and I'd just missed a metro, which is only twice hourly at weekends) felt a bit of a let-down. But at least the through sail-rail ticket from London was fine for the metro as well as for the subsequent regular Dutch trains.
IR has never ever a garantee that it remains valid on all routes as on start of the year-railways may also simply close down lines or nowadays more often they get run by another non-IR accepting company, like much in PL or CZ. The ´official´ website of IR is full of errors, mistakes and incomplete info. Its just a good introduction to the world of non-UK state and strikeprone railways.