Let the speed of the second train be v km/hr. Since both trains have the same length L and are moving in the same direction, relative speed = (60 - v) km/hr = (60 - v) × 5/18 m/s. The first train catches the second and leaves it 120m behind, so total relative distance covered = L + L + 120 = 2L + 120 meters. Time = 18 seconds. Therefore: Distance = Relative Speed × Time. However, we have two unknowns (L and v) and only one equation. But wait - catching means starting together, then overtaking by 2L (lengths of both trains). If they start side-by-side and first train overtakes by 120m, then 2L + 120 = (60 - v) × 5/18 × 18. This still has L as unknown. The standard approach assumes the 'catch and leave behind 120m' means relative distance = 120m only (both start together). Then 120 = (60 - v) × 5/18 × 18, giving 60 - v = 24, so v = 36 km/hr.