In message , Paul Scott
writes

Right - well done for finding those pictures. I wonder if that more modern
looking ramped access visible in the various aerial view sites, eg Multimap,
just provides an access into the RH of the two smaller tunnels shown he
http://www.geograph.org.uk/photo/366747
to reduce its effective length, ie just to get under the railway?

I actually gave the wrong URL for the first photo, but you have quite
rightly spotted the correct one.

Yes, I think the ramp simply goes down to the old foot tunnel, which is
why someone earlier in the thread couldn't see where what appeared to be
a pedestrian subway emerged.

AIUI, there is a footpath above the pedestrian tunnel (which provides a
viable alternative to the long, unlit tunnel), and the ramp was put in
to take pedestrians down to the one bit of tunnel that is still used,
beneath the railway line. The Google satellite image shows this path
quite clearly.

AIUI the main flow will follow the path of least resistance. I imagine
under the conditions of heaviest rainfall the Crane drains surface run off
from a vast area?

I suspect that, like a lot of London's smaller rivers, it takes a huge
amount of surface water when the drains can't cope in heavy rain. The
Environment Agency's flood map shows the banks of the Crane (and its
tributary, the Yeading) virtually all the way from Northolt to
Twickenham.

Interestingly, the same map indicates little or no flood risk for the
mill stream on Hounslow Heath, which I presume means that it is now
pretty much cut off from the main channel. This again could have been a
contributory factor to the bridge collapse on the main channel.

Another possibility is that the smaller tunnels were some sort of flood
channel of course, as originally built.

At least one of them might have been - I can't see any reason for two
parallel foot tunnels. If so, their closure (probably dating back to
when the marshalling yard was built) would certainly have exacerbated
the problem.

--
Paul Terry