Back In The Day -- before the Mojave National Preserve -- it was fairly easy to fan the Cima Sub. Driving South out of Nevada, exit Interstate 15 @ exit # 286, turn East on Nipton Rd., ( is the town still for sale ??? ) then South on Ivanpah Rd.; a few miles South take the right fork in the road. You are now on Morning Star Mine Rd., which will take you to Cima. The road becomes Kelso - Cima Rd., and is within feet of the tracks all the way to Kelso. from Kelso West, the tracks run through the sand dunes and the next decent access is at Afton Cyn. Road. ( I did find out though -- Back in 1989 -- that U.P. had paved their GATE - FREE maintenance road through the sand dunes !!! I don't know if it is still accessable or advisable though. ) If you are limited to pavement vehicles, you have to head North from Kelso on Kelbaker Rd. to Interstate 15 @ Baker, CA. The drive will pretty much kill any chance of seeing the train again before Cajon Pass though.
I have also driven Nipton - Moore Rd., South from Nipton. It becomes Bryant - Cima Rd. South of Moore and continues into Cima. This road was kinda - iffy Thirty years ago, with busted pavement & pot holes. I don't know if this road is even passable, or open, nowadays.
For an easier Hit - n - Run deal, you could go to Nipton, ( the top of a decent grade out of Stateline, NV. ) then head back to I - 15 and head West to Afton Cyn. Rd.; a decent dirt road down to the campground, the Mojave River, & the tracks.