When using a single track for multiple trains between two stations, it may happen that a train must wait for a long time before it can use the track, especially when the track is long.
The reason is that only one train is allowed on a single block of tracks to prevent train collisions.
Partly doubling the track
To solve this, you can split your long single track into several pieces, as shown below.
Partly doubling the track.
The long single track between both stations is partly doubled. Trains going to the energy plant travel on the bottom line, and trains traveling towards the mine use the top line. In this way, trains can pass each other.
The signals on the double track part are all single direction only.
Completely doubling the track
Partly building a double track so trains can pass each other was fruitful, but it still allows for delays when trains don't synchronize well enough. A logical extension is to double the entire track.
Completely doubling the track.
In this way, there is one dedicated track for trains going towards the energy plant, and another dedicated track for trains going towards the mine. It looks like:
In addition to doubling the tracks, each track is now split in multiple blocks with uni-directional signals. In this way, trains can travel after each other at the same track which greatly improves efficiency. Also, the energy plant now has its own depot, which looks nice, and provides good servicing to the trains.