I've seen some signals with a "G" on the mast to indicate "grade", permitting a train to pass a red signal without stopping.
Looks like the NP used a yellow rectangle for a permissive signal. I never knew that. First time to see one on a semaphore like this.
Quote:
larry zeutschel
AMT #10 W. of Livingston, MT
All the original merger partners are represented on this early version of Amtrak's North Coast Hiawatha. NP power and baggage, SP&S coach, GN coaches, a BN painted diner, and CB&Q sleepers.
Coming off the Bridger Mountains the train passes by a staggered semaphore. The yellow rectangle was an added feature the Northern Pacific used if there was plenty of forward visibility. The engineer could pass the signal without stopping watching out for a train ahead.
AMT #10 W. of Livingston,MT by
larry zeutschel, on Flickr