The given values on the problem are:
fo = 2000 Hz
f = 2115 Hz
v = 340 m/s
vo = -2.00 m/s
Since vs, or the final speed of the train is asked, we used this equation:
f = fo (v+vo)/(v-vs)
Substituting the give values:
2115 = 2000(340-2)/(340-vs)
By algebra:
vs = 20.4 m/s