Tube lines interchange

[Gav1n]

Can anyone think why there's no interchange between the Hammersmith&City/Circle/Metropolitan line and the Northern/Victoria line at Warren Street?

If you look at the map (http://www.tfl.gov.uk/tfl/images/Zonal-Map.gif) you can see the lines overlap but you can't get between them without walking above ground. This makes some journeys needlessly complicated (e.g. Camden Town to Baker Street)

[archbishop]

They might overlap on the map but the depth of each line might have made it uneconomic to join them up at the time of building. Which was probably at least 100 years ago.

[brachiopod]

The lines now known as Hammersmith & City, Circle, Metropolitan and District Lines are the oldest lines on the underground system. Constructed in the mid-to-late 19th century, they are relatively shallow, constructed by the 'cut-and-cover" method, involving digging a deep trench and roofing it over. For this reason they generally run beneath roads and are known as the sub-surface lines.
The Northern and Victoria lines are deep lines and constucted by actually tunnelling through the earth. The Northern line was the first deep line to be built (in stages) around the early 1900's, whilst the Victoria was built in the 1960's.

At the section of the Underground in question, all Hammersmith & City, Circle, and Metropolitan line trains share the same pair of tracks, (rather than being three distinct lines as such), and these run E - W just beneath the Euston Marylebone Road.
To link the Victoria with another deep line would have been comparatively straightforward (such as at Warren Street), but the costs to construct an interchange with the sub-surface line at, say, Euston Square, would have been prohibitive in this constricted area , involving digging up major roads and probably demolishing buildings.
In addition, in order to allow for cross-platform interchange at Euston with both the Northern Line branches, the Victoria line had to cross over itself twice - the southbound tube crossing over the northbound between Highbury and King's Cross, and back under again just south of Warren Street. There was insufficient room to put the crossings any closer, and certainly no room for an interchange with the sub-surface line.

With these complications, and with the proximity of Euston Square itself, it was deemed sufficient to connect between these lines at ground level rather than having a dedicated interchange.

[ianess]

Can I second that, In A Pickle?    That was one of the best and most informative answers I`ve seen on AB in a long time.

Well done brachiopod!