This again takes us back to the notion of there not being “slow” trains. It’s the same agent that handles passengers. Coal, oil, people, they’re just names for cargo that have absolutely no bearing on how the engine treats its urgency. All cargo is created equal.
Another possible workaround to have “slow” trains being bypassed by “fast” trains is to use diesels for “slow” trains and run a non-electrified track parallel to the mainline.
