Although there have been successful trials of freight container trains from China to Germany, there is doubt about the future of the service because there is such a large imbalance of traffic. There have also been seven trains from Shenzhen to Prague, but it is difficult to get enough traffic in the other direction to make a cost-effective round-trip. Some operators say that five times as many containers travel from east to west as from west to east. Railfreight operators are targetting air freight, not sea freight competitors. Rail is considerably quicker than seafreight, especially when ports in China become congested, but is cheaper than airfreight.