For now, it remains unclear quite how far any passenger will travel on future HS2 services – whether reaching Euston or on high-speed tracks all the way to Crewe. But however short the journey, an exclusive peek at HS2’s embryonic carriages reveals travellers will be enthroned in “the best seats of any UK trains” – even, arguably, in the toilets.

Conscious of discontent with the “ironing board” seats in some of the newest trains introduced to Britain’s railways, HS2 are putting passengers’ backs and bottoms centre stage.

The trains won’t be built until 2026 – but in a dedicated room in the manufacturer Alstom’s factory in Derby, full-size wooden mock-ups of the train interiors to the contracted specifications have been constructed for diverse user groups to test.

The standard seat will have more legroom – a “best-in-class” 87cm – and be able to recline without impinging on others’ space. It will also feature built-in reading lights, easily accessible charging points and plugs, a fold-up table and a rack to hold electronic devices for easier viewing.

“Our ambition is to make this the most comfortable seat on trains in the UK,” said James Dawson, a senior rolling stock engineer at HS2 Ltd.

Cushioning will be a priority, although Dawson stressed space constraints would bring compromises.

Simon Aslett, the rolling stock director of the West Coast Partnership Development, which will operate the trains, caveated that testing groups’ preferences varied: “We’ll go with the consensus – but we know someone will find it uncomfortable.”

Those who have to leave their seat to answer the call of nature are well catered for, particularly any parents struggling with young children. As well as changing tables in every toilet, the new loos will include innovations such as a pull-down seat to secure toddlers, and a pull-out rack to store bags just above the level of any suspiciously wet floor.

An information screen by the mirror will update on the train’s progress for passengers’ who might not hear public address announcements.

The toilets may also be cleaned more regularly than British passengers expect, according to Aslett, who has been studying Italy’s high-speed service. “Italians won’t put up with some of the conditions we accept in the UK,” he said. Funny signs and talking toilets are unlikely to feature, he indicated.

There will be slightly more headroom than usual, with the train’s mechanical equipment taking up less ceiling space than on most existing trains. A dedicated section of the train will have bookable space for four bikes, and also spaces for unfolded pushchairs and buggies.

The tweaked final designs will go into production in 2026. HS2’s user consultations so far have resulted in them adjusting poles and overhead racks, as well as removing deep cup placeholders on tables on cleaners’ advice: “Kids will mash jam into them,” Dawson said.

Like the rest of HS2, the trains have attracted some controversy – including a legal challenge to the procurement process – and costs have been affected by political indecision and changes.

Across the original HS2 network, designed to run to Manchester and Leeds, the train carriage floor would be at the same height as the new HS2 platforms, allowing wheelchairs – and buggies and suitcases – to roll straight on.

“Level boarding makes such a difference. It’s one of the best things about this project,” Dawson said. That will be possible at Old Oak Common and Birmingham: but with the trains increasingly likely to have to run on existing conventional tracks, all the carriages will now be fitted with retractable steps for non-HS2 stations where platforms are lower.

The 54 trains will be manufactured across three sites in England in a £1.6bn contract: the shells at Hitachi’s Newton Aycliffe site, the bogies (wheels and axles) at Alstom’s facility in Crewe and final assembly at its historic Derby factory.

The publishing of the mock-ups comes as fresh speculation swirls around the wider HS2 project, since it was suddenly scaled back by Rishi Sunak a year ago. Labour is yet to confirm the expected funding of HS2 tunnels to Euston station in central London, and whether and how additional rail lines could be built north of Birmingham to Crewe and Manchester to address the capacity issues on the west coast mainline when HS2 trains start running.