Railway heatmap.

It would be an amazing quality-of-life feature to have a Speed Heatmap Overlay for the railway network. The overlay would display a rolling average of train speeds over the last few in-game months, color-coding the tracks from red (slow/congested) to green (full speed).
This would give players a quick, visual way to diagnose traffic bottlenecks and immediately see if upgrading a junction, adding a signal, or changing a schedule actually improved their network efficiency