The train's speed is 90 km/h, which is equivalent to \(90 \times \frac{1000}{3600} = 25\) m/s. The time taken to cross the tunnel is 40 seconds. Let the length of the train be L meters. When crossing the tunnel, the train covers the tunnel's length plus its own length. Therefore, the total distance covered is \(400 + L\) meters. Applying the formula: \[ \text{Distance} = \text{Speed} \times \text{Time} \] we get: \[400 + L = 25 \times 40 = 1000 \text{ meters}\] Solving for L: \[L = 1000 - 400 = 600 \text{ meters}\]