I had extended my metro line by branching out from a track turning into a depot (effectively making that track a branch), and subsequently creating a triangle intersection (I'm pretty sure that's not how you call it, but basically a T-junction in which every track can go to the other two). What happened next was puzzling -- I had never toggled the visibility of any line: the path leading from the last station (before the extension was made) to the depot, and back, disappeared, cutting the line off from the depot. A message also shows "(Train) can't find route to next (destination)". A similar message appears if I manually try to re-add the depot. Upon finishing the extension, pathfinding to the new stations (which, in my (the end-user's/player's) field of vision, is a connected straight track) also fails.
What is happening? Have I done something wrong or forbidden?

In this pic, I selected one of the waypoints to the right and it was giving me that message. The message isn't shown here, though; the screenshot probably cropped it out.
What is happening? Have I done something wrong or forbidden?
In this pic, I selected one of the waypoints to the right and it was giving me that message. The message isn't shown here, though; the screenshot probably cropped it out.