Fresh from the factory, Britain’s latest hi-tech train glints under the powerful lights of the depot. With its aerodynamic nose-cone and sleek racing-green carriages, the machine stretches into the far distance.

“This is a very different train to the 40-year old trains it’s replacing,” says Sam Fisk from manufacturer Hitachi, and our guide to the North Star depot near London’s Paddington Station. “We’ve imported technology from Japan and our design centre has made sure the train is shaped so it cuts through the air.”

Some 200 of these Class 800 hybrid units – capable of running on both overhead power and diesel – will eventually replace Britain’s 40-year-old High Speed Trains. They recently made their debut on the line between London and the west of England and will soon be rolled out on the East Coast Main Line to Scotland. That line is a route made famous by world record-breaking steam engines such as the Flying Scotsman and 126 mile-an-hour Mallard.

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Inside, the train has that new car smell with unblemished rows of spotless seating. At the front is a futuristic cab with computer screens and arrays of switches, looking like it was transplanted from a Space Shuttle cockpit.

“It’s a digital train,” says Fisk. “Drivers came in to test the design, so it’s single comfortable seat at the centre and even has a handy cup holder.”

That’s all very well, but where you have drinks you eventually need toilets. What are they like?

Since the end of the 19th Century – and in the US even earlier – most long-distance trains have been fitted with toilets. They’re a necessity for any long-distance journey but most passengers do their best to avoid them.

“There’s been terrible toilet design,” says railway author and historian Christian Wolmar, who’s travelled the world by rail. “They range from the very basic on Indian railways – where’s it’s not much more than a hole in the floor – to modern toilets, which have the dodgy trick of the door opening by itself.”

Ah, yes… the self-opening toilet door. Anyone who has ever used a British train toilet has almost certainly experienced the excruciating horror and embarrassment of the automatic door slowly rolling open for fellow passengers.

I can’t understand why the design is so difficult and complicated – Christian Wolmar

The problem is down to the design of the buttons, and complicated instructions (in English only), requiring users to press one button to close the door and then another to lock it. To make things even more confusing, the switches are often back to front.

But this isn’t the only issue with train toilet design…

“They tend to have a completely unfathomable system for the sink – the water comes out, air comes out and soap comes out all from different bits hidden under a shelf,” says Wolmar. “Inevitably you turn on the hot air before the water - I can’t understand why the design is so difficult and complicated.”

It’s issues like this that the designers of the new high-speed trains were told to address. With the trains commissioned almost 10 years ago, they’ve had plenty of time to get it right.

“The government set the specifications and one of the feedbacks was that the standard of the toilets had to be raised from the earlier fleets,” says Fisk. “We built a wooden mock-up of the whole train and have had more than 200 groups of passengers on board to give us feedback – then we went to the factory and built the real trains based on what they said.”

So, did the people get what they asked for? BBC Future decided to put this “better” train toilet to the test.

We’ve made sure the locking system is a lever and you have to push that lever to the right – Sam Fisk

Each train has five toilets on board – three are similar in size to aircraft toilets and two are designed to fit a wheelchair and include a fold-down baby changing bench. We chose to investigate one of those.

First impressions are good - the toilet is spacious, clean and well-lit, with grey and cream walls. It even has inset wooden panels and two coat hooks.

“We made sure that we had wheelchair users come in here, you can turn a wheelchair, we also had people with guide dogs and people with babies,” says Fisk. “It’s really important that the spaces and angles are right.”

So, onto test one: locking the door.

“Being able to lock the door was the number one feedback,” says Fisk. “We’ve made sure the locking system is a lever and you have to push that lever to the right.”

A red sign lights up and a woman announces from a hidden speaker that “The door is locked.”

So far, so good.

We tried to simulate a hotel toilet – it’s nice, it’s bright – Sam Fisk

Next, the toilet itself. All seems to be in order – there’s toilet paper within easy reach and the bowl flushes with considerable power. So forceful in fact that rumour has it Nasa was inspired by bullet train toilet design for the International Space Station.

Toilet waste goes into a tank under the train. Like many others around the world, some older trains in the UK still dump waste directly onto the tracks. “Do not flush in the station” signs are, apparently, largely ignored.

Finally, the basin - a pushbutton arrangement that might not entirely please Wolmar but seems to function well enough.

This new train toilet is hardly revolutionary but it goes a long way to addressing flaws in train toilet design. “We tried to simulate a hotel toilet – it’s nice, it’s bright,” says Fisk, “we wanted to bring a nice feel to the place.”

Out on the mainline, the new Hitachi trains have had a number of teething problems. Passengers report hard seating, leaking air conditioning and issues with onboard catering and timekeeping. One train recently broke down completely, leaving hundreds stranded for six hours.

Still, at least the toilets are well designed.

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