Found a few more when I widened my search to just natural gas locomotives. Some items of interest:
POLA switcher test: www.glmri.org/downloads/lngMisc/PortofLongBeachLNGforLocomotives.pdf
Progressive Railroading (2014): [
www.progressiverailroading.com]
Railway Gazette (around 2014): [
www.railwaygazette.com]
TRAINS forum (2008): [
cs.trains.com]
SCAQMD Board Meeting Item (2016): [
www.aqmd.gov]
What I couldn't find was a good comparison of locomotive emissions and operating issues between *NG and Tier 4 diesel in its varying (AdBlue and Not AdBlue) forms. There must be a credible study of that by somebody. Webdigger?