One of the most interesting features of TTDX is the signalling. In many other railroad games, signals are simply lacking, instead trains randomly choose where to go and if two trains get on the same track they can just go through each other without being damaged. In TTDX, placing signals correctly is almost as important as building the track in the first place. Without signals, one cannot build complex railway networks, which are essential for high profit.
Signals are built using the appropriate tool from the menu. At the first click, a two-way signal will be built, clicking this signal again changes this into a one-way signal, further clicks change it into a one-way signal for the other direction and back to the two-way version. After placing the actual signal, one can change the signal type using the graphical interface.
When holding Ctrl while clicking a signal as when to change the signal type, the GUI window opens. One can select the signal type by simply clicking the appropriate icon, with the greyed icon marking the current type.