NNG SLRG Survival

When the Union Pacific has a line that is declining in revenue, they have enough income from the remaining railroad to absorb the loss of income. When the Union Pacific sells off a section, like the Alamosa line, and the SLRG takes over, and it has some revenue but the revenue starts dropping, what happens. I have always been told that as long as there is one customer, the railroad must continue to service the line, but when the cash isn't enough to pay the bills or maintain the rails and equipment, what happens, does rail line revert back to the original owner or do things just go belly up. I thought the SLRG started the passenger service / tourist train to supplement the freight operations but it looks like they have just about given up on the passenger service and freight operations appear to be stagnant or drifting lower. What is going to happen.