As others have mentioned, the "UPY" reporting mark designates locomotives that are typically used in yard and local service (although it doesn't prevent non-UPY units from being similarly used), as a means to solve the problem of running out of available numbers. The program began in 2000. "UPY" appears in various places on the locomotive, and the majority also have a "Y" ahead of the numbers in the number boards. Many "UPY" units of 1500 horespower or less also have red and white diagonal reflective striping on the ends.
You can see a list of most of the "UPY" units numbers on Don Stack's
UtahRails.net web site: [
utahrails.net]