I would suggest to add tunnels. It could even be tiered as for longer tunnels you could need to research TBM and even make one at the vehicle depot.

To add on to this suggestion, one possible implementation could be a tunnel-boring train engine designed specifically for underground expansion.

The train would need to carry concrete blocks or tunnel segments to construct tunnel walls as it advances, as well as track sections so it can lay new rail behind itself. It should also require loose material cars to haul away excavated rock, dirt, or debris from the boring process.

To make this system practical, the rail network would need multi-directional track controls, allowing the boring train to reverse direction and return to a loading/unloading station. At that station, it could unload rock and dirt, then take on more concrete segments and track pieces before continuing the tunnel.

This would create a more involved industrial process where tunnel construction requires planning, logistics, material supply, and waste removal rather than simply placing tunnels instantly. Some kind of modification of the attached photo.

In addition to my last comment, the boring engine could also separate extracted minerals for loading into rail cars, essentially functioning as a mobile sorter. As long as the boring train is configured for the minerals it is expected to encounter, it could automatically sort them as it advances. If the engine detects minerals it is not configured to process, it would stop before reaching them so the train can be reconfigured or redirected.