This paper discusses principles for identifying and appraising the impact on GHG (green house gases) emissions from electrifying transportation. We are primarily interested in the road-to-rail case, but the principles apply to other cases as well, both inside and outside the transportation sector. The underlying motive for the paper is an ongoing debate regarding how and to what extent investments in rail infrastructure influences GHG emissions and how to adequately consider the effects in cost-benefit analyses (CBAs) of such investments. Various approaches have been proposed.