A Tunnel

We are part way towards an answer.
The smaller tunnel is indeed an original C&SLR bore and the nearer section is an enlarged tunnel. You can see the smaller inserted segments that have been added to increase the diameter. You can also see how the shape of the tunnel has been altered in the process and is no longer circular.
BUT, what happened here?
It is an old photo, but it is not contemporary to the reconstruction - it was taken years later.
The tunnel is still there and, if you remove all of the clutter, it looks the same today.
Any further ideas?

What a brilliant answer. It makes complete sense.
Totally logical, but wrong.

Pardon my going off topic, but what is one of these?

Q8 wants a go!

Here's his suggestion

"Re : the tunnel pic. I would hazard a guess at two things. Firstly it was used as a compressed air lock during the reconstruction. Or it was used as an air-raid shelter."

Have him join up then

I can see Harsig checking his track diagrams

I class anything between Euston and Clapham Common as the City side (sometimes even as far as Morden). As a result, Stockwell does qualify but, no this is not the old incline tunnel.
All of the posted suggestions have been based on what you are seeing in the picture - an enlarged tunnel joining onto an original tunnel. Stockwell and Borough Junction are therefore perfectly sensible options as reconstructed tunnels would have met original tunnels at those points. As a suggestion, may I advise that the actual location is not too important. What I am looking for is what has happened here; and that was certainly influenced by something that happened elsewhere (clue). It might also help to consider what I said in the text that accompanied the picture (another clue).

London Bridge, the passage way from platforms to the old IMR, now a relay room.

Not passageways at London Bridge but, this is close to a station. That probably will not help though.

Oh waland, you are so warm. If no-one else gets it, I think I will have to give it to you as being closest.
It is actually not a test stretch, but think along those lines (or parallel to them).
Does anyone recognise what the gap is between the two different sizes of tunnel or notice anything unusual about its shape?

I understand the chief feature of the enlargement work, and the one which made it economically possible was the retention and re-use of the original cast-iron lining, in which by doing so gave the new tunnel a slightly irregular shape. My guess is that this a sort of mock up tunnel to test the theory.

Is it at Stockwell?

Well done everyone, some of you were so close and all of the others posted answers that were completely credible.
What you are looking at was part of a running tunnel that was enlarged to this point while the trains were still running. It would be part of a Northern Line running tunnel today had not something happened.
With much of the enlargement completed, an incident occurred in November 1923 and the line was closed to all traffic. This not only allowed reconstruction work to continue day and night, but it meant that additional unplanned realignment could take place. In this instance, the company decided to alter the approach to Elephant & Castle station and put the platform on the other side of the station tunnel.
What happened here is that they just stopped work and abandoned this part of the enlargement. This piece of running tunnel was then cut off by the concrete plug which is immediately behind the camera position. The present day tunnel comes in alongside this one and to the right.
The large space between the two sections of tunnel is a shield chamber – the point at which work was stopped to allow the shield to be removed. The shield that had been used here was of a special design that allowed it to remain in place so that trains could run through it during the shortened traffic hours. To allow this to happen, the track levels on the enlarged and unenlarged sections had to be in-line with each other. That is why the shield was 'off-centre' - as you can see from the shape of the chamber - all of the space being at the top.
I actually find it very strange that they bothered to remove the shield at this point, when it might have been just as easy to have continued the short distance to the station where it would have popped out of the wall!
The result of this is that we now have this little time capsule recording a point where one of the special enlargement shields was operating. On one side is a section of tunnel that it completed but which was never used and other side is the section of tunnel that it was working towards before being told to stop.

I personally found CSLR's photo and explanation quite fascinating, I hope he's got more in the tank so to speak, well done.