I found that trains and trams suffer for very strange pathfinding.
In this junction, trams arriving through red brick tunnels had in schedule to go stop visible by dialogues. However, instead it they went through buried tunnel beneath house with two floors house with red roof (that is to the south of cursor) - even if they had free way and alternative way was much longer.
And also, sometimes trains try to find alternative way to scheduled station - even if any alternative is much longer. Also, it may lead to stuck stations because trains affected by wrong path finding ignore choose signals.
Sorry, your description is a puzzle. The most common cause for a detour are ither too many 180 degree bends (which count as 10 curves) or, more common, a wrong signal. The latter could be even accidentally on a crossing. Use the unblocking tool to check the signal overlay arrows. (And beware that they will not work correctly on crossings.)
There were three lines going through those tunnels and then to that station. Two lines were going as they should. One line not. So, it really was not caused by wrong signals.
And when I buried that tunnel, trams began to go as they should.
Please repost the screens with the route you expect and it took drawn in different colors. That way it should be more clear.
You can use a simple image editor to draw the route in.