JOSM Mapping From Scratch Best Practices-Multiple .OSM's or One?

I am mapping my hometown historically with OHM and it is a blank canvas. I have highly detailed maps of the railroad, and have started there. Once done, I want to do roads, then buildings.

Should I have the data split amongst multiple .OSM (railroad, roads, buildings etc.), or just draw it in one? If I do draw in multiple .OSM's, how do I share nodes between them? What should I do if I want to do other towns?