The stuff under the track is a 30cm foam tile setup made by kyosho japan. It's made for their dnano cars. I bought it (2nd hand) to use as a demo track with my larger cars. Just happens to work well as a barrier to stop the real fx cars when they leave the track.
Bonus is that when the cars go wide they sense the dark foam tile and keep the car turning right
next time I will put some white card down on the inside so the cars turn left to return to the track.