The challenge here is specific to this line - other lines (like Circle) already have AC on the trains - mainly because they’re absolutely huge trains by comparison
London 2 main types of underground line: Subsurface (Circle, District, Metropolitan and Hammersmith and City lines), and tube. The subsurface lines were constructed by cut and cover and were made for steam engines. They are much larger than the tube lines. The Tube lines are tunnelled, and designed for electric power only.
It's not really the size of the trains that allows AC on the subsurface lines, it the fact that the lines are well ventilated to allow steam and smoke from steam engines to escape.
Even the article specifically points out the size of the train as an issue
> To fit air conditioning into the trains has been rather an interesting challenge, as air conditioning units are large and tube trains are small, and Siemens Mobility has taken several ideas and put them together to create the space needed.
