You always keep pairs of carriages, connected like this: +----------------+ +---------------+ | T| | | +----------------+ +--

You always keep pairs of carriages, connected like this: +----------------+ +---------------+ | T| | | +----------------+ +---------------+ O O O O where 'T' marks a toilet. You build your train from a bunch of those cars, and now it does not matter how you connect them, you'll always have a toilet within a car's range, plus at worst the additional cost of crossing into the next car (since you can't put the toilet between two cars, there's a slight asymmetry).