I used various types of signals at portals to warn of hidden crossings in mountains or behind backdrops, at mainline/reversing loop boundaries to indicate polarity, and at block boundaries to indicate a train ahead as well as the direction of the train ahead (is it approaching or receding?). I also used dummy signals next to my uncoupling areas to indicate the best place for couplers to be uncoupled.
The block signals for the six mainline blocks were made with Carl Goldberg's #462 90-degree mounting brackets. I painted them black, drilled out the holes, and inserted yellow- and blue-stained 3mm 12-volt lamps—which I CAed (what a verb!) into place. The blue is simply a block boundary marker connected to the current block's (before the boundary) track power and the yellow is connected to the next block's track power. Lacking in prototypicality as they do, these signals may be replaced someday. The other six block signals, for direction indication on the next block, are #2100 two-light green and red ground dwarfs from New Jersey International. If a train is backing up towards my block, this signal indicates that. In truth, all I really needed were the boundary signals, and warning signals for hidden crossings, but . . . I got carried away (I admit it).
The NJI (#2100s again) polarity signals at reversing loop boundaries on the upper levels show green if the polarity is aligned and red if it isn't. This keeps trains from suddenly being thrown into reverse.
The same NJI signals were used in one other way: train detection. Each of the four hidden staging area tracks have Opto-Sensors right where engines should stop. I keep the green lights of these signals always lit up but the red ones only come on when a train is detected. I find I can control the trains effectively and safely using this method even though I can't see them, as long as I run the trains slowly enough and as long as I pay close attention to the red lights and stop when they light up. The sensitivity on the sensors is set with wheels on the DT-4. Because the layout room's windows let in sunlight during the day, lighting conditions in the room were variable. So I installed a light bulb right above the sensor area (pointed right down at them), which made all other lighting sources irrelevant. This didn't affect layout lighting since the staging area where this light was added is behind a backdrop and therefore out of sight.
The only two alternatives I had to the light sensors were tethered throttle controls so I could walk from the control panel to the edge of the staging area and watch trains arrive and depart, or a mirror mounted on the ceiling at a 45-degree angle that would allow me to stay at the control panel and see train movements behind the backdrop. It was an easy decision to choose watching signal lights over the other options.