Nothing leaves until the line ahead is clear
Get every train out. A train can only go if its own track is clear all the way out of the yard.
Every train has a track running out of the yard, and it can only go if that track is empty all the way. Tap the ones that can leave.
A train's colour is the way it is facing, so you can sweep the yard for everything heading one way without reading a single nose: right, left, up, down.
Most tracks run straight out. A few turn a corner on the way — those are drawn the whole way, with a diamond where the points are and a matching arrow in the fence where the train actually comes out. Those are the ones worth checking twice.
A wrong tap costs a lamp. The train lurches forward, thumps into whatever stopped it and stays there, and a red cross shows you where they met. You get three lamps for the whole run, not three a yard — and clearing a yard without a single wrong tap puts one back on, up to three.
A train with a gold number has to wait till the end. Clear everything else first, then take the numbered ones in order — 1, then 2, then 3. The ring round its number is red while it is still waiting and green once it may go, and tapping one too early costs you nothing; it just peeps at you.
The four levels are the same yards, just starting further down the ladder of sixteen — EASY begins with small yards and no junctions, EXPERT drops you straight in with long trains, corners everywhere and four numbered trains. The yards keep getting harder as you clear them whichever one you pick.
You can never get properly stuck. A train that can go stays able to go however long you leave it, and a shunt that would make a yard impossible is quietly refused. If you stop tapping for a while, one train that can leave will start to breathe.