The 'square foot gardening' site, although specifically for sfg, will have answers to your soil question. Another source is the Texas A&M U cooperative extension service site.

Happy gardening.

Railroad ties are not a good idea for a raised bed where edibles will be grown. They are soaked in creosote, a recognized toxin that will leach into the soil, and are considered a hazardous material. If they are already in place, you need to line the interior with a heavy mil plastic and extend it down below the soil level 4-6 inches.

You do want to get rid of the grass - personally I'd opt for manual removal before using a herbicide, or you can layer the bottom with a good thickness of wet newspapers before adding soil. This will smother the grass or any weeds and will decompose naturally.

Rather than 'topsoil' which is a very generic term that can mean anything and usually amounts to just low quality fill, look for a 3-way planting mix at bulk soil suppliers or local nurseries. You don't need peat moss and I'd avoid it - it really doesn't add anything and can excessively acidify soil and will repel water if allowed to dry out.

Assuming the ties are laid on the surface of the ground and not buried, you will need about 7 cubic yards of soil for each bed. (Calculate square footage - 10x20+200', times the depth (12" or 1') and that will give you 200 cubic feet. Soil is sold by the cubic yard so you need to divide that by a factor of 27 (27cf to 1cy) = ~7.5cy). You can add the leaf mulch but mix it in. You don't want to fill the beds completely to the top, either, so that's why I went with the lower number.

And do contact the local TAMU extension office. They will have a lot of good information about growing plants in your area and when to plant, etc.

Have fun!

If, I assume they are, the ties are very old, there is zero creosote to volitize/leach into the soil. If you have concerns about this issue, don't plant root crops within a foot of the ties as lateral movement of substances in the soil is very minimum.

I respectfully disagree. Creosote is extremely persistent in the wood and will continue to leach for many years. When I purchased my home and garden 25 years ago, there was a retaining wall and steps made of railroad ties - who knows how long ago they were constructed and they were used by the railroad for years before that. Every summer, before we finally removed them, in warm weather they oozed thick, black, tar-like goo....creosote. And creosote's ability to leach into soil and groundwater is well documented - the more moist and warm the conditions, the greater the amount of leaching. And since raised beds tend to be in full sun and frequently watered, leaching will occur.

Creosote is a known carcinogen. The risk is sufficient enough that the EPA does not approved creosote-treated railroad ties for residental use. And they are illegal to be sold or recycled in some states, mine included. It's just not worth the potential risk and it's easy enough to mitigate by simply lining railroad tie raised beds with plastic or metal flashing.