TL;DR
When hovering over a train with "no path" show an overlay of all the rail pieces that it can reach similar to how hovering over other trains shows the planned path.What ?
My suggestion is to use an overlay like the one used for showing the path of a train when hovering over it. But instead of showing the path show all the rail tiles the "no path" train can reach. Also helpful would be showing the reverse direction starting at the trains destination in a different color.For the train map view it would also be nice to show the same information by coloring the rails. The world view only has a limited zoom and train networks are usually bigger than the visual area.