There is a place in an underpass/bridge west of Grayditch where we can walk through the wall and get stuck under the road.
We just walk against the left wall:
Then this will happen a couple of times while we walk against the wall:
That happens in this pillar/thingies:
But here is the place where we can go through the wall, it's in that corner:
Then we get stuck under the road like this:
It's hard to explain where this is so I got a pipboy map screen: