Driving an icon into the next century
The length of the intended lifespan of the trains also presented some challenges. "We needed to think about materials that will 'wear in', or look better with age, rather than wear out," Priestman says.
"What's interesting with transport is that if you design something that looks too futuristic, the likelihood is that it will very quickly look dated. So you need to strike a balance with the design, create something iconic to take passengers into the future whilst avoiding clichés."
As there is currently a range of trains from different periods running on the Underground, Priestman and his team were able to cherry-pick the most important and recognizable elements of past designs, and innovate around them.
"If you look at the London Underground compared to other subway systems, the trains are more comfortable," he says. "We would have liked to have created more floor space but it became apparent early on in the process that this space was required for elements such as air cooling."