I've seen lots of people asking around on how to make custom RMC rails here (myself included), and figured I'd direct you all towards this very helpful video by TheCodeMaster:
I tried it out myself on one of the preset coasters, and it looks great! And for extra points, I switched the color to blue with an external photo editor (since I know people are getting sick of the orange/red

) Still figuring out the cross ties though, that's the confusing part.