Railroading is simply placing your needle between your threads before you pull it through the fabric to ensure the threads lie next to each other smoothly instead of on top of each other or twisted. If you've never done it, it may slow you down quite a bit, but once you get the hang of it it will go much faster. You'll also probably develop a "feel" for how to manipulate your needle as you're stitching so that the threads lie next to each other on their own and you don't actually have to constantly railroad.
I railroad both bottom and top legs of stitches, which results in nice chubby stitches and full coverage. I don't like to see a lot of fabric under my stitches; I prefer more of a "mosaic tile" look instead of an "xxx" look.
I railroad all my stitches, no matter the color, but I particularly like the results when stitching with black. To me, if something is supposed to be black, it should be black... not flecked with little peeks of the fabric underneath. Here's an example of railroaded black stitches.
I hope this post is helpful to someone. It's been helpful to me to be reminded to share tips and how-to's, and to not assume that everyone already knows.