Each train you dispatch loops its route forever as a ghost, so every new delivery has to be threaded through the shrinking gaps left by all the trains you sent before.
Pick a coloured delivery, then draw its train's path one cell at a time from source to sink. Once dispatched, that train loops its path endlessly. Each later route must avoid colliding with any ghost at the same point in its loop, route every delivery within the train budget to win the level.
Drawing a route cell by cell from source to sink is the path-laying transit puzzle Mini Metro made popular.
The original constraint is periodicity. A dispatched train loops its path forever, and each later route must avoid every earlier train at the same point in its cycle — which is the problem railway signalling exists to solve.
That makes your own solutions the obstacles. A clean early route is also a permanent occupation of that track at that phase, so the levels tighten not because they add walls but because you keep adding them yourself.
Loop Logistician is a spatial routing puzzle where your own past solutions become the obstacles, blending path-drawing with timing-aware planning. It is an original, free browser game on PlayPendium.