The second wave front

In front of the train:The second wave front, however, is not emitted from the same place because the train is moving forward. After the first wave is emitted, the train has 1/250 s to travel forward before the second wave appears. How far does the train travel during this time? Again, we can use the formula for distance to calculate how far the train has moved.
x = vt = 20 * (1/250) = 0.08m
In other words, the second wave front is 0.08m closer to the first than if the train were still. The new wavelength L' is therefore 0.08m shorter than the original wavelength L.
L' = L - x = 1.372m - 0.08m = 1.292m
This new wavelength, 1.292m, is the wavelength the observer in front of the train actually hears.This is he
wavelength we will use to calculate the observed frequency. Our equation will be
f' = v'/L'
The velocity, v', in the above equation is the same as v. It is 343 m/s. The sound waves move through the air at 343 m/s regardless of the motion of the train. Our final calculation is therefore
f' = v'/L' = 343/1.292 = 265 Hz
The observer hears a frequency of 265 Hz, slightly higher than the original 250 Hz sound produced by the train.