I guess that the case is not described properly.

assert.equal(fastTrain([[5, 5], [4, 2]]), 6);

In the image the trains crosses the first station on the speed of 3 and continue moving on. The speed is satisfied for the first section but it is not appropriate for the second section. I guess the description of the second case is not finished.

From: https://js.checkio.org/mission/fast-train/solve/