On some of my passenger trains I use a function only decoder that drives a rear Gyralite. For years I used a function only decoder on a C&NW push-pull cab car that had a rotary beacon and headlight in forward and the red markers came on in reverse. I just programmed it to the same address as the lead engine.
I have since replaced that with a Soundtraxx Soundcar so now I have a horn and bell, plus air conditioning compressor sounds, too [Y] along with the beacon, headlight and markers I can also turn the interior lights on or off.
If it were me, I’d use an FL-2 and you can have the white light on in reverse or have a Mars OR Gyralite when the train is forward.
https://www.tcsdcc.com/Zen/index.php?main_page=product_info&cPath=8&products_id=45
Once it is set up like that you never have to mess with it and you can keep all the engine functions simple.
[edit:]
I should note that, while operating as a train, there were regular electric marker lights. Most photos I have seen show the Pyle cat-eye type although somewhere I have seen the Aerotrain with a pair of big 'ol Adlake “non-sweating” markers.
http://rr-fallenflags.org/nyc/nyc1001drl.jpg
Railroad operating rules do not recognize an oscillating signal light as “markers” therefore the Aerotrain could not be designated as a train without the additional marker lamps displayed.
Have Fun! Ed