Stability of South Norwalk CT 4-track trackbed?

[Eleanor Weiss]

While watching rail activity on the South
Norwalk Tower (Connecticut) railcam, a
question has been bugging me.

The cam covers a 4-track trackbed
between the junction of Washington and
Main streets and the Norwalk River, (where
the old rail bridge is being replaced.)

This section of trackbed is raised above
street level so that vehicle traffic can pass
under bridges.

Google Maps street view images show
that this section of track bed is supported
at the edges with large slabs of rock or
concrete forming vertical walls.

But what I'm curious about is, what sort
of material is inside those rock or concrete
walls. Is it a combination of various grades
of rock, gravel, earth, and ballast?

If so, why doesn't the material inside the
walls gradually settle, due to weight and
vibration from the trains, as well as winter
freezing and thawing? Wouldn't that cause
the walls to be pushed out, and the tracks
themselves to sag, eventually?

Or is there some sort of structure inside
which maintains tension to keep the walls
in place?

Thanks to anyone who has an answer.

Eleanor Weiss