The toilets on the first BR trains I knew in the '60s were primitive affairs; the bowl emptied directly onto the tracks, and relying as they did on speed to disperse the contents there were urgent pleas never to use the toilet whilst the train was standing in a station. In the early '70s continental trains were a delight of discovery; not only did the bowls empty on to the tracks, but if you held the discharge lever open you could, in the right conditions, stand mesmerised as the clearly visible track ballast whizzed past beneath you. These days they all have holding tanks and the ballast is cleaner than at any time since the running of the first railways.
Of course, vacuum toilets and holding tank systems have their downsides. When the first Inter City 125s were introduced to the UK, the high air pressures created at speed in tunnels caused an unfortunate 'blow back' that left many passengers covered in ... confusion. And at the weekend a French chap had his arm sucked down the bowl of a new TGV while trying to retrieve his mobile phone.
Boats can still discharge toilets straight into the sea. My own is equipped with the ultimate in boat toilets, the Lavac. It's never failed, despite the efforts of all womankind to jam it's 1.3/4" outlet with profligate swathes of bum wipe and even more unmentionable articles. It's as reliable and maintenance-free as the BR train toilets of the 60s.
This cannot last. Already all boats on inland waterways must have a holding tank and 'pump out' their waste at designated stations. Soon, the fingers of regulation will seek to extend this to sea boats, thus depriving thousands of oysters and mussels of their sustenance and causing inconvenience to owners. But like the smoking ban at sea - a nonsense which even Brown's State can't enforce - this measure is doomed to failure.
I have spent years persuading my fishing guests to pee in a bucket and rinse it over the side rather than just leaning over the gunwales and unzipping (men always scorn the Lavac for some reason). Particularly on the Thames through central London. However, if the holding tank becomes compulsory, the terrace at Westminster may be greeted by bare buttocks precariously hung over the side and worse. Only some MPs would be interested, I suspect.