Here is an obligatory roster shot of Norfolk Southern's "Southern" Heritage Unit. I think this is one of the sharper looking heritage units that Norfolk Southern has. Note the "K" on the cab behind "8099". This is part of a number check system that Southern used.

To find the letter, do the following:
1st digit + (2nd digit times 2) + 3rd digit + (4th digit times 2).
If doubling a digit results in a two digit number (9 x 2 = 18), then add the two digits together to get a single digit. So 9 x 2 = 18, 1 + 8 = 9.

So, using this formula, (8 + 0 + 9 + ((9x2=18), (1+8=9), so 9)) = 26.
Subtract that number from the next even multiple of 10. So 30 - 26 = 4.
Then, find the letter that corresponds to the calculated number:
0 = A,
1 = F,
2 = H,
2 = J,
4 = K,
5 = L,
6 = R,
7 = T,
8 = W,
9 = X

From the calculation above, the single digit number we calculated was "4", and "4" matches "K", which is exactly the letter that is on the cab!
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