Sorry if this is too far astray from the main topic but it's interesting that the article says: "It is the first major expansion of the Tube since the Jubilee line in the late 1990s." Comparing the two however, well... There's practically no comparison in terms of scale.
ethano92345 , norbitonflyer , Whilst what you both have said is true, it's important to remember the context of how these extensions came to be.
The Jubilee Line Extension (JLE) was to help the already popular DLR in regenerating the Docklands, an area that had lost over 150,000 jobs by the time the London Docklands Development Corporation (LDDC) came into being. The JLE had to be on a big scale as it was being put through an area the size of the borough of Islington that had been historically isolated and was set for major development with projects such as Canary Wharf and the Millenium Dome to name a few.
The Piccadilly's history with Heathrow is markedly similar to Heathrow's history regarding its own expansion as it was mostly a case of build as you go. The Piccadilly was expanded just as the airport needed more terminal space and the public transport was moulded around it as demand increased.
Whilst the Northern lines extension is comparable on scale with that of the Tube's arrival at Heathrow, it's core aims being part of a wider urban planning/regeneration scheme are like those on JLE with a raft of new developments in an area that ironically has been laced with railways since the 1800's but has had poor connections due the industrial history of the area. Much like the DLR and JLE put places like Canary Wharf and North Greenwich (actually Greenwich Peninsula) on the map, places like Nine Elms and Battersea look set to become areas that are more regularly talked about with arrival of the Northern line.
an area that ironically has been laced with railways since the 1800's but has had poor connections due the industrial history of the area.
It might have been cheaper just to put Queenstown Road and Battersea Park stations on the Tube Map - although QTR in particular needs a bit lot of sprucing up.
(There have been grand proposals in the past to replace the interchange at Clapham Junction with a Battersea Central, incorporating both BPk and QTR, as well as platforms on the Overground and South Eastern lines that pass across the site, allowing the closure of some platforms at Clapham Junction to improve alignments, and therefore speeds, through the area)