I wouldn't use maple for a post.
I wouldn't use a railroad tie either. You do not know what species of wood the tie was made out of since they buy any and all hardwoods that are big enough to cut into railroad ties. The railroad relies on them resting on gravel so water drains off of them quickly. They where not made for ground contact, and when used as posts, or retaining walls, or garden borders, the bugs get into them and hollow them out. For years, they look normal, but if you hit them just wrong, they crumble.
Spend the money for a ground contact treated post. Mix concrete and pour it around the post, and then add more so that it slopes up out of the ground, and around the post. The goal is to make sure that water runs away from the post and that water NEVER EVER sits at the base of the post. For line posts, you are fine dumping concrete into the hole dry and letting it cure on it's own, or even packing dirt, but for a gate post, you want the maximum strength possible, and that's achieved by mixing it with water. Mixing makes a significant difference in strength, as does not using too much water. The stiffer the better!!!