I found a few things you may want to fix/improve.
First of all, at Wrongingville, you have 2 double tracks going north, that meet again a little further north. Obviously, there is room for reducing the amount of track there.
too_many_tracks.png [ 2.39 KiB | Viewed 2127 times ]
Secondly, at Witleigh woods, you don't need 2 depots. There are only 2 trains on the entire track there. For 140 ton/month from the mine, the trains are too long. OpenTTD likes lots of arrivals of trains, so shorten them by 2 wagons or so.
Thirdly, all over the place, you have way too many signals. Lets start with a few simple rules.
a) On simple straight track (no junctions nearby), signal spacing must be a bit larger than the train length.
b) Don't have signals on adjacent tiles, always at least 1 empty tile between them.
I removed about 50 signals this way
Fourthly, at Boundingbury Heights, you don't need 3 unloading platforms, 2 is enough. Also, the depot behind the station is costing you money. Trains arrive at the platform, want to be serviced before unloading, and go straight to the depot. After service, they can't go back to the station, so they make another full round, with making any money. Move the depot to before the station.
You also don't need 2 tracks leading to that station, you can remove one of them.
Fifthly, at the other side, at Nantditch Mines, you don't need 4 platforms, so I suggest to remove the northern one, so you can remove the tunnel as well.
Sixtly, the tracks to the depot on the incoming line there are just silly, don't have incoming and outgoing traffic cross each other. Remove the tracks, and have incoming traffic and outgoing traffic each have its own depot(s). Likely, this gives you so much better performance, you can remove the 3rd platform at Nantditch Mines as well.
Seventhly, Old Grintforth woods produces 35 tonnes wood / month. You can't afford to have 1 train for that, let alone 2. Remove both trains and the station + tracks.
Eightly, the same holds for Dunham Valley, both trains run losses, just remove it all.
Finally, the main dish, Gruhaven park station. The "before" picture shows the current situation.
The main idea of the change is that on the high ground, trains get into the station, while at the bottom ground, trains leave. Just that change was enough to clear the queue from the north-west (to Malwell-on-sea). I connected 4 platforms to the north-east branch (to Nantford), but that wasn't enough.
For this reason, I extended the high ground so I could add a signal after the bridges, but before the junctions. this minimizes the time that a platform is unused. I also connected the north-east branch to all platforms. That did the trick, both queues are gone now. Some special care is needed to make sure leaving trains can always clear the platform and the junction (this is why there are no signals there).
PS While checking the "after" picture, there is some unused track on the north-west branch, you should remove that (or rather, not build it).