A worker strikes a steel railway rail with a hammer…

Step 1. Speed in steel. $v = \sqrt{\dfrac{Y}{\rho}} = \sqrt{\dfrac{2.0 \times 10^{11}\ \text{Pa}}{7850\ \text{kg/m}^3}} = 5050\ \text{m/s}$, to three figures.

Step 2. Travel times. Through the rail: $\dfrac{1200\ \text{m}}{5050\ \text{m/s}} = 0.238\ \text{s}$. Through the air: $\dfrac{1200\ \text{m}}{343\ \text{m/s}} = 3.50\ \text{s}$.

Step 3. The gap. $3.50\ \text{s} - 0.238\ \text{s} = 3.26\ \text{s}$.

Why steel is so fast. Steel is about a million times stiffer than air, but only about six thousand times denser. The ratio $Y/\rho$ is far larger, so the wave is about fifteen times faster.