Describe each line in terms of its most significant / largest population origin and destination? Thus removing arbitrary allocations of "down" or "up" (words that in themselves could have been better chosen, being counter-intuitive for the vast majority of the country that lies North of London) for circumstances such as the lines between Inverness and Aberdeen, or Newcastle and Carlisle, or anywhere running East-West.
The key element here is safety. If the broken rail (or whatever) being reported is on the Up Slow at Madeley why would describing it as the Southbound Slow or London Slow be an improvement?
And, as I've said earlier, the "vast majority of the country" has no relevance - what matters is that the staff involved can identify track correctly.