Guardrails like these pictured here from ontracks are fine to add to your slot car layout and look good in their stock configuration. All brands of track have their own guardrails and most slot car sets these days of guardrails as standard for most of the kits.
The guardrails basic aim is to make sure that a de slotted car does not fly off and break which is a very bad thing considering the price and detail of the models.
The guardrail systems have not changed over the years, they are a made from a tough flexible plastic that can absorb most impacts. The guardrails have little tabs at the bottom that clip under your slot car track.
Track Borders and Rumble Strips
Everyone knows that real life race circuits have rumble strips or track borders as most slot car hobbyists call them. Most of the slot car brands have ready-made track borders like the ones pictured from ontracks here and are more than adequate for most slot car track designs. The idea of the track border is to take the advantage that the outside lane has in lots of situations the outside lane can use full power through a corner and the guardrail acts to keep the car from de slotting on the other hand if you have track borders in place the outside lane can’t use full power through the corner due the car sliding and eventually de slotting.