Three identical train cars, coupled together, are rolling east at \(2.0 \mathrm{~m} / \mathrm{s}\). A fourth car traveling east at \(4.0 \mathrm{~m} / \mathrm{s}\) catches up with the three and couples to make a four-car train. A moment later, the train cars hit a fifth car that was at rest on the tracks, and it couples to make a five-car train. What is the speed of the five-car train?