"A train going through a tunnel."

Meaning

The train enters the tunnel — invisible for a while, then emerges on the other side. This degree is the invisible form of emotional transition. People here move deeply without showing it; a year later they may be entirely different people, but others notice the change only at the end. Cancer's inner life is here — silent transformation inside the shell. Patience in the tunnel is required.

Shadow

Not exiting the tunnel; staying continuously in darkness; turning into depression.