While I was wanting to include Man, western UK, Wales, and maybe even Misty Island, due to the map restraints of 4096x4096, I'm going to have to exclude those again. I doubt any Railway Series purists are going to be upset over this.

I might be able to squeeze a bit of Barron-in-Furness in.
So far my heightmap building skills have shown I've gotten better, but I still need more improvement. My technique for laying in riverbeds is sound. The main problem is the Edwards III maps only give 20 levels of height, ranging from sea level to 2000 feet. The highest point on the map is Culdee Fell at 2049 ft. The good thing is that default OpenTTD settings put inclines from one level to the next at 50 feet, so it will be simple. The hard part is that the way the map is laid out, each defined level is 200-300 feet in difference. In order to make this a good, usable map I'm going to have to make up 4-6 height levels in-between and try to figure out how the landscape should be shaped.
If anybody has any good descriptions of each location, that would be really helpful.