If you take two random stations, the odds are 3/1 that at least one of them (the later one) is in the second half of the alphabet. The odds of the next one down the line being even further towards the end of the alphabet are less than half. As your chain progresses, the number of available stations gets shorter and shorter, so the odds against get longer and longer.The odds are not 50/50 each time.
Letter frequency makes no difference, as it merely the stations' positions in the sequence that matters.