One way possibly to reduce the current through the #151 semamphore is as follows:
1) Power the #151 semaphore from an "accessory transformer", independent from the train-powering transformer.
2) Put a diode, a resistor, and the activating contactor in series with the power to the solenoid.
3) From the resistor, by-pass the contactor and the solenoid coil with an electrolytic cacapacitor.
When the contactor is "open", power from the accessory transformer will charge up the electrolytic capacitor to full voltage.
When the contactor closes the circuit to the solenoid, the full charge on the capacitor will "kick" the solenoid into action, but the resistor will limit the amount of "holding" current which the solenoid will receive.
I would guess that the resistor would be somewhere in the range of 5 ohms to 10 ohms (5 watts), and the bigger the electrolytic capacitor, the better (maybe 1000 microfarads, 35 VDC). Experimentation with different values may be necessary.
If I succeed in attaching the diagram, it may make this idea clearer.
