When a railroad such as Frisco, buys a steam locomotives class,1500s for example, why do railroads have to operate them in one jurisdiction instead of the entire railroad?
Especially in the days of steam, RRs designed/ordered locomotives for specific jobs. The 1500s, for example, were designed to pull passenger trains over Ozark hill-and-dale topography, and their characteristics reflect that intent. Later in life, it was not uncommon for these specific assignments to go to even newer engines, and so older one went on to other assignments, maybe the same kinds of trains on other divisions, maybe different kinds of trains, maybe just whatever came along. But, when new, most were order for specific tasks, and they were assigned to those tasks as long as they could carry them out and were serviceable. The details can get fairly complicated, but this overview should give you a basic answer.
Sometimes certain classes of engines would be restricted from certain lines because they were a bridge class too heavy.
