John,
The red marker is a flag for the flanger mounted in front of the pony truck. The flag shows the engineer if the flanger is up or down. The D&RGW installed Priest flangers on many (but not all) K-28s, K-36s, K-37s. All ten K-36s came from Baldwin with the mounting brackets for the flangers, but the D&RGW did not get around to installing them until the late 30s and early 40s.
The flanger on 483 at Chama.