Not sure what tool you are using but sometimes you can draw a line and then tweak it with bevels, embossing, stroking, whatever to get some good effects. If you just want it to be a simple line of some kind then I think you need to figure out the order of significance. Highways bolder than roads? Railroads probably need some kind of cross hatch unless the name of the track gives it away. Some detail may not matter depending on what level of detail you plan. You might make rails like roads but use a different color then you could do highways with solid line and roads with dashes or something.

Take a look at some of the finished maps and see how others have done roads. That might give you some ideas as well.