I was looking at the roster of the BNSF in the 2008 Locomotives Roster & News and the engines numbered 5718-5747 are listed as AC4400EVs. Where the 5600-5717 engines are regular AC4400CW. Whgat does the EV stand for, what makes these different from regular Ac4400CWs and why did the BNSF order them. I have seen at least two.