January Quiz - Day The Last

The above quiz is a previously run version. the following is (I think) today's version.

Question:
Four locations, find the link between them, and the two stations that could be fifth in the set

Location A:

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Location B:

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Location C:

(click for a larger version)

Location D:

(click for a larger version)

In which case a good guess at the other two might be Wimbledon and Richmond.

And the last set, Olympia and Edgware Road. 

So Wimbledon for A as ends of the District Line? With Edgware Road and Olympia as potential fifth, as innercircle said?

Hang on, the colours in the location A and location D photos have changed from the first posting to the second, is that a clue or an error?

I have two theories
1, line termini that have a rail connection, but I had to dismiss that as you said Wimbledon isn't one of the stations.

SO
2, five line termini in five different counties. So Surrey is covered by Richmond (so no need for Wimbledon as well), Essex is covered by Upminster (so no need for Walthamstow Central as well), and Ealing Broadway for what was Middlesex.

So, three to get:
Woolwich Arsenal for Kent
Amersham for Bucks.
Watford Junction for Herts.

The link is unrelated to political geography, and while rail connections aren't completely unrelated I don't think that will help you find the answer.
The change of colours (also in location B) is as domh245 states - just a modification to try and disguise the locations a bit. Sometimes it works sometimes it doesn't. The second posting is the unmodified version with the colours as recorded by the camera.