Speed (m/s) = distance (metres) ÷ time (seconds)
For Track 1: 0.2 ÷ 2 = 0.1m/s
For Track 2: 0.2 ÷ 2 = 0.1m/s
For Track 3: 0.6 ÷ 6 = 0.1m/s
For Track 4: 0.4 ÷ 6 = 0.07m/s
The train on Track 4 had the slowest speed because it's got the shortest speed and it's covering less distance per second therefore it is slower.