The NYMR picture appears to show a vacuum toilet. I wonder where the power to operate such a system will come from, functional ETH isn’t common on most heritage line stock as steam locos can’t provide it.
Weren’t the mark 3 sleepers built with conventional flush toilets that emptied into tanks due to their extended dwells in platforms? I think the BedPan class 317 units also had a similar set up that was subsequently removed.
That would seem a much more practical arrangement for heritage lines. The point of vacuum toilets is that they use much less water, meaning smaller tanks for water and waste. This would be less of an issue on heritage lines with their lower ‘volumes’. Vacuum toilets are also horrendously complicated and would be a nightmare for heritage operations to maintain.