Neo magnets could pull off the weight issue. They can be made strong enough to be a physical danger. (that is your safety warning against blood blisters or worse, they are no joke when strong enough)
Possibly stop actual rear ending of two cars in a lane if used with opposing fields in a bumper for repelling collisions?
Track mods could likely give two directions on slots. I also recall old slot cars with two guide pins, one at the front another at the rear.
Google Manco undershelf layout; and watch the trains run upsidedown!
(hopefully still out there as the forum Dale has passed away)
Lee, have you tried an ho powerpack to get a steady speed? I did it with my AFX/Auroras long ago because I broke my trigger control. (It was too fast, but a resistor was had from Gramps. No racing, but it kept me busy looping till a controller could be had. Older Aurora were much slower; I'mpotential the magnet quality was better during their AFX era.
Slot cars were another reason I almost left lionel for ho long ago.
After decades of "wishes" and various ventures by Lionel into other toy development; Lionel slot cars in O seems like a no brainer, long overdue. It may not have cash cow fad-like potential, but seems a safer bet than many of the past failures, and at least doesn't require a heavy helium tank and batteries to operate! (google " Lionel Helios 21" ) While the scale might have been wrong, at times I still ran mine with my trains looping right along side of them. I would have loved a RR crossing. With cast trains, the cars would have been wrecked sooner but oh well. (prototypically leaving hardly a scratch on the cast loco if they collide