TL;DR
Color the last guide box showing train length when placing signals or stations.What ?
You can define "Train visualization length" which is helpful. However, the last marker should stand out from the rest to allow an at-a-glance visualization if the full length has been met.Why ?
If you're trying to space signals for a full train length, the visualization length is helpful in that it allows you to count the box outlines to see if the full train would fit, but if the last car was a different color or stood out in some way then we'd know instantly that the full train would fit. I would find this to be a significant QOL improvement.A quick glance at the one below shows no red box, so you instantly know the whole train length won't fit without having to count the boxes.