I have a suggestion to improve the signalbox UI.
Now, the train toolbar shows only signals that can be built without any signalbox. More signals are shown only after clicking on already built signalbox. Usually the player knows which signal he wants to build, but has no idea which signalbox is necessary for that.
I propose that relevant signals are shown when the signalbox is selected in the toolbar. Thus user would get quick overview what signals are available with which signalbox without building all of them.
That is an interesting suggestion, but is not as easy to implement as at first appears, as that would require a new toolbar function of having buttons visible but not able to be selected or used. It may also be confusing for players who may find it difficult to understand why they cannot use buttons that have just appeared in the toolbar.
There could be a warning dialog saying: you have to build the signalbox before any signals.
Or the signalbox builder ui could be similar to factory/curiosity builder. It has more space to convey info about signals.
I would support that a signalbox is builded from a window like factory/curiosities window. Maybe, when you click on a signalbox (In the normal rail window), the additional window appears?
The first option is likely to be confusing to players because some signals would be able to be built with some signalboxes and some not, and players could not see without actually trying to build them which ones can be built.
The second would require a very great amount of work (many weeks dedicated just to this), which is not a task of sufficiently high priority to consume this much time. If somebody else would like to code a better UI for this, however, I should gladly incorporate it.