I set up a very simple train service connecting a colliery to a coal-fired power station. Initially all works as expected, with the colliery supplying large volumes of coal to its station, a single train taking coal to the power station's station, and the power station consuming the coal. The power station consumes coal faster than the train can deliver it so the power station's coal stock always drops to 0 while it's waiting for the train to return with another load. There is always enough coal at the colliery's station to fill the train up immediately upon arrival. As I said, all as expected.
Eventually though, coal starts piling up within the colliery and only a small amount goes to its station so the train has to wait to be 100% full even though there is plenty of coal within the colliery. The power station's stock will drop to 0 and it has to wait considerably longer for its next supply.
So, help, what's happening? Why does the large volume of coal within the colliery stop being transferred to its station when it was happening initially?
(I'm running Simutrans 112.3 Ex 11.35 with Pak128.Britain-Ex-0.9.1.)