Having found contact lenses in implausible circs (obviously not THOSE circs) I think of finding the knife as an "improbable possible" - the key variable is when it falls from the step. If we assume it fell from the step only just before the train left the tunnel, it can only have fallen on the narrow strip between train track and tunnel wall; effectively, the area of Nicola's search is limited to a strip about three feet wide (assuming it didn't fall off when the train was passing one of those maintenance hatches that are cut into the sides of tunnels for personnel working on the line to shelter in) by the length of the distance between last time seen and end of the tunnel.