Answer
The train is a distance of $737~m$ away.
Work Step by Step
We can let $v_1 = 5790~m/s$ be the speed of sound in steel and we can use the travel time $t_1$ of the sound waves through steel to write an expression for the distance:
$d = v_1~t_1$
Note that $t_2 = t_1+2.1~s$. We can let $v_2 = 331~m/s$ be the speed of sound in air and we can use the travel time $t_2$ of the sound waves throguh the air to write an expression for the distance:
$d = v_2~t_2 = v_2~(t_1+2.1~s)$
Since the distance is the same, we can equate the two equations to find $t_1$:
$v_1~t_1 = v_2~(t_1+2.1~s)$
$(v_1-v_2)~t_1 = (2.1~s)~v_2$
$t_1 = \frac{(2.1~s)~v_2}{v_1-v_2}$
$t_1 = \frac{(2.1~s)(331~m/s)}{5790~m/s-331~m/s}$
$t_1 = 0.1273~s$
We can find the distance $d$:
$d = v_1~t_1 = (5790~m/s)(0.1273~s) = 737~m$
The train is a distance of $737~m$ away.
