Conemaugh Road & Traction circa 1956
Use .040" styrene for the street surfaces. Cut it to fit the inside/outside of the rail. You'll need to support the curb with something to avoid having too high of a crown (the slope from the center of the street to the curb). Raising the street .080" at the curb would give a reasonable crown.
Use 1/16" strips between the rails. Evergreen 1/16" tile makes a good approximation to the rubber fillers used at real grade crossings.
You can try something the real railroads often do to keep the flangeways clear. Glue or spike a guardrail just inside both running rails (use a NMRA gauge to make sure the guard rails don't get too close to the running rails), then pour whatever goop you use for paving material in between the guardrails. You can make a narrow scraper tool out of styrene to level the paving with the top of the guardrails. Also, it may be wise to use rail slightly shorter than the running rails to make the guard rails, i.e. use code 83 guardrails with code 100 running rails. This will ensure that the running rails can still be kept clean with an abrasive block or whatever you use.
Tom