While it delivers rich three-channel sound when connected to a TV, it also lets you create a true five-channel surround setup pretty much cable-free using a pair of Sonos Play:3 wireless speakers.
The two differ only in how they get their highs out, with the S350 including a pair of squat stereo speakers and the S400 offering a three-channel "air surround xtreme" soundbar.
The pair had different visions, with Watkin, who wanted to build a Channel tunnel rail link to France, seeing the Metropolitan as a mainline railway, while Forbes preferred shorter extensions.