bxl-forever
In fact it is not really a tunnel.

It was tagged as "tunnel=yes" in OSM for convenience but, technically, the railway line runs at ground level and the new hospital has been built on top and covers it.

In a perfect world the railway line should have been tagged at level 0 with the hospital area/building at level ≥1.  But this would be very error-prone once people would start drawing the inner details of the hospital.

We have many similar cases, e.g.
https://www.openstreetmap.org/way/589841814
https://www.openstreetmap.org/way/371914167
where there is no real tunnel (i.e. a structure that was built underground) but ordinary infrastructure covered with something on top.  More stuff to clean up one day.

If that’s okay for you, I suggest we close this note.