Thanks guys, more progress to post in a few minutes
I just checked, rails dont fit into the slots. I have some Code 70 Rail, and even when trying to force it end from the end, it is very hard to do so. This being said, they could be modified to accept metal rails by doing a little more kitbashing, but i wont be going that far on this one anyways. If left plastic slots, you will not be able to run a powered locomotive in the mill and you would need to use a spacer car to get a car if needed. The way the base plates are orientated right now however, the slots are only long enough to hold 2 coil cars each, so this shouldnt be a problem with not being able to get a powered locomotive in there.