Powder River Coal Basin - Trains Magazine

Sounds like you'll be heading up I-25 to get from Cheyenne to Casper.  Just short of Casper is Douglas, WY.  Catch WY Hwy 59 North at Douglas.  It more-less runs along the west boundary of the basin for 100 miles, all the way to Gillette.   Just a bit north of Douglas, you'll cross over the BNSF-UP shared mainline.   Get on Google Maps and find Bill, WY.   Coal trains are easy pickins there.    Just north of Bill,  is Logan Hill (4 track main line.)     A few miles North of Bill, National Forest Road (NF 942) goes east (right).   Follow it about a mile to where there is an overpass, above the 4 track main.  You can see one of the load-outs from there.  Find the Jct of Wy Hwy 59 and 450.  Go a few miles east and you'll pass over the tracks.   You can get on Hilight Rd there, which follows the tracks north and south. 

If you want to see a serious bit of the basin, go to Reno Jct.  Last time I was there, they still did tours at Black Thunder Mine.   Jacobs Ranch mine is nearby and there is new load-out under construction.  

Search this forum for Powder River Basin as there have been others who asked the same question previously and folks gave detailed answers.