Since the Alco 251 is still made and sold. Do they meet tier 4 standards and if so why do we not see and locomotive rebuilders using this engine?
Fairbanks Morse has not developed any tier compliant engine upgrades for the 251, not even Tier 1. The railroads still operating Alco power in the United States are all Class II and III carriers or industrial operators and they are not legally required to meet the same emissions restrictions as Class 1 railroads (unless they buy brand new locomotives).