A 20% increase in efficiency doesn’t mean the train uses less power though. Ramming more punters in makes them more efficient, accelerating quicker makes them more efficient, dumping power back into the wires makes them more efficient.It is from Siemens [1] and is widely quoted by the press. Not that I blindly accept such figure, but whether it is an over-conservative estimation is a completely different discussion from this post.
[1] https://www.mobility.siemens.com/global/en/portfolio/references/metro-london.html
Modern trains will use more power in general because their traction motors will draw more energy to provide more motive effort. And they will have more onboard draws, such as air con, more computers, more passenger info screens etc.
It’s why you see many new fleets requiring a lot of infrastructure improvements.