Having found a reference to a tunnel under the road at Sainsbury's, I had a nose round with the camera today.
Sure enough, you can see the two ends and some of the infrastructure joining the tunnel to the Fallowfield path.
From the west the tunnel is approached by the side of the buildings on the railway bed.
The entrance can be seen, blocked by gates.
From the east the giveaway to its location is the cycle track in the middle of the vehicle ramp
which connects to the gap in the kerb...
Behind the wall is the cycle route
which would have entered the tunnel here.
The designer has removed his version from his web site, so sadly we don't know if he thought this was a good idea or not.
This all begs the question, just how much money was spent on this tunnel and who ended up paying for it?
Update: a follower on twitter has found the text for the deleted web page on the Wayback Machine, so we can now see what he wrote, though the pictures are now sadly lost.
View Fallowfield Loop Tunnel in a larger map