I reversed engineered the Whistle relay a few years ago.
1720-X050 R1 Whistle Relay
W1 Red AC input W2 Black AC input
W3 Blue Whistle motor positive W4 White Whistle motor negative off Q1 Collector
C1 10 micro F Q1 Emitter to Base C2 470 micro F decoupler across Red and Negative
D1 4002 Red AC wire to R3 and W3 R1 10K ohm
R2 10K ohm to Q1 Base R3 50 Ohm between D1 to W3
Q1 KSP-13 NPN Darlington Transistor: Emitter on W2, Base on R2, Collector on W4
Here are parts for the
600-8152-050 Diesel Horn Circuit
IC1 MSM 4069RS IZ77S
Q1 2N5172 Q2 CD9014
Q3 CIL 232 C1 470 micro F
C2 10 micro F C3 EL 100K
C4 El 100 473J R1 22K ohm
R2 1K ohm R3 10K ohm
R4 270K ohm R5 10K pot
R6 33K ohm R7 10K pot
R8 33K ohm R9 70K ohm
R10 74K ohm R11 17K ohm
Normally, the fault is the Darlington Transistor fails. They are cheap to replace and also found on many of the early Lionel Reverse units.
Looks like the K-Line replace the transitor with a relay instead.
I don't think the one resistor is 75K. While Lionel used different resistors on different boards, they were matched. 75K and 7K is not match. Try the 10K resistors. G