Yeah Its easy for cars to get launched over them, but its usually due to the placement of them on the track. Also one thing you can do is double stack them back to back to create a barriers where needed. Plus if you design layouts properly then you could reduce these problems. Try to avoid placing rails perpendicular to the racing line. For instance a gradual sweeper or carousel on or off the strait is a good example. Most driver's will not jump over this type of section because they are not driving directly towards the barriers at perpendicular angles. I also used other elements such as a heavy floor mat at apexes to discourage corner cutting to close to the rails. Hope this helps.
There's also a lot of other track materials you could use such as old fire hose (contact your local fire dept, they give them away alot). Also I heard of CRC's click system, might be another option.