What's your question? Ask

You can leave them as mulch. Here's an article that will answer your question:
https://www.gardeningknowhow.com/garden-how-to/mulch/mulching-with-leaves.htm

This is an actual case of rust! This happens when there is a high iron content in, either, the stones, or the water or soil touching them.

From here, it is advised to use soap and water to avoid damaging the concrete. If that doesn't work, then citric acid can. Just be cautious with this, since it can corrode the concrete if used in excess.

Western red cedar does best in well draining soil, but some sources said it would tolerate clay soil. They do prefer a moist to wet soil, so you have that as well. I would build up the area with some high quality soil, it doesn't really need to be enclosed in a raised bed. Eventually the good soil will work its way down into the poorer soil.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/yard-drainage-problems.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm

My first question would be whether you have composted the horse manure. Fresh manure in your garden is far too hot for anything to grow unless you've tilled it into the soil much earlier. This should be done in the fall or winter prior to the following spring planting. An even better idea is to compost it separately before using it as fertilizer. Here's an article that will help with that:https://www.gardeningknowhow.com/composting/manures/horse-manure-compost.htm

Next, is your garden in full sun? Most vegetables need 6 to 8 hours of sun per day. Are you watering and fertilizing properly?

Even with the new soil, I would recommend a soil test before planting in the spring. Here's a helpful article:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/test-soil-for-pests-and-diseases.htm

And more:

https://www.gardeningknowhow.com/edible/vegetables/vgen/the-vegetable-garden.htm

https://www.gardeningknowhow.com/edible/vegetables/vgen/problems-growing-vegetables.htm

We have some articles that will help to explain what to do in preparation for a raised bed. These will be very helpful:

https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-vegetable-gardens.htm

https://www.gardeningknowhow.com/edible/vegetables/vgen/layout-options-for-gardens.htm

https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-bed-soil-depth.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/best-soil-raised-garden-beds.htm

The article below says it is best just to replace the railroad ties. I read recently the new railroad ties today aren't toxic.

Here is more:
https://www.gardeningknowhow.com/garden-how-to/info/are-railroad-ties-safe.htm