A railroad car moves at speed \(v_{\mathrm{i}}\) on a track toward three other railroad cars at rest on the track. All the cars are identical, and each stationary car is some distance from the others. The moving car hits the first stationary car and locks onto it. Then the two hit the second stationary car, lock onto that one, and so on. When all four cars have locked onto one another, what is the speed \(v_{\mathrm{f}}\) of the four-car train?