London's entire fleet of surface line stock is being renewed with a single fleet. Some of it dates from 1960, most of it from around 1970 or 1978/79. All has been refurbished in the past ten years and looks perfectly presentable.
Of course there are advantages in having new trains and a single uniform fleet, or ought to be. They can use less less electricity through regenerative braking, and they should also cause less wear and tear on the track and need less maintenance.
But will they? And how long will it take to realise the savings and recover the cost of the investment? It's just a question.