The interiors of all the tunnels are marked with regular number markers. This is inside the tunnel 26 – the larger “10” is in stone from the GIPR days, while the modern version is the angled plate with reflective text that is universally found in all the tunnels.
I think these markers are called “stations” and they help to accurately locate a point inside the tunnel, in the absence of catenary structures or permanent way distance indicators.
The 2.156 Km long tunnel 25C has 69 station markers – so they are located fairly close from each other.