I think Woodland Scenics has a dry transfer set that incudes RR street markings but I've never had much success with these since any inperfection in the roadway inevitably tears up the dry transfer. What has worked for me is 1/2" white Helvetica stick on lettering sets from Office Depot and and a set of Elmer's fine tip paint markers from Walmart. The letter set is about $4.00 and has enough "R"s for seven pavement markings. The Elmers paint pens are the best I've ever used, with truly opaque paint and a very fine point that makes it easy to control. They come in a set that of five that includes white and yellow, so you also have a way to do pavement striping too.
I lay down the two "R"s on the pavement so they look right for the size of the rood. I use a small piece of plastic sidewalk as a straight edge to draw the two lines at the top and bottom and the "X" in the middle. You might want to practice this part first on some scrap roadway material. Once you get the hang of it, you can make some pretty nice pavement markings for a reasonably cheap price.
I've been trying to find decals or dry transfers for the pavement markings at RR crossings. They would be relatively easy to mask and paint, just wanted to see if anything premade was out there. I tried Microscale and Walthers sites to no avail, I may not be using the right search terms.
Rick