The GP31ECO is another variant in EMD's ECO locomotive conversion catalog. The locomotive is the first EMD ECO to utilize a 3,000 hp 8-710ECO prime mover. The locomotives are EPA Tier 1 compliant. So far two conversions have been conducted, both for Southern Peru Copper. #70 and 71 were rebuilt from GP40 cores, possibly of UP lineage. The locomotives emerged from Progress Rail's Tacoma shops in March and April 2013. 

Decoding this EMD ECO Designation

GP- General Purpose (core)

3- 3,000 hp

1- EPA Tier 1 emissions

ECO- EMD ECO locomotive