Technical specifications:
- power supply: 9VDC.
- consumption: 55mA max.
- dimensions: 1.98 x 1.05 inches

How it works:
IC1 is configured as a frequncy generator with the frequency characteristic dependent on R5, R6 and C4.
Pin 3 of IC1 is connected to capacitor C5 for coupling the sound signal to loudspeaker.
IC1 has a reset pin (pin 4) when this pin is lo (less then.4 of a volt) the IC will be shut down.
Pin 4 controls the train's whistle sound by multi-vibrator circuit (TR1, TR2).
Multi-vibrator (TR1 and TR2) is configured as a simple frequency generator.
TR1 and TR2 will alternate one by one.
If TR1 is on, the collector at TR2 has the voltage, causing IC1 is not working.
IF TR2 is on then IC1 will generate its programmed audio signal.
Operational characteristics of the multi-vibrator is dependent on R2, R3, C2 and C3.

FK235 SCHEMATIC

FK235 Circuit Layout