I do believe the arrangement is with certain larger dealers to honor warranties and even as before I am sure they will hold some product currently being made to support warranty repairs. Even Lionel does this. The parts continuation is actively being pursued.
So what does this mean. If you are a risk taker buy from anyone. If you are risk adverse buy from a reputable dealer and thoroughly test out the product once you get it. Then enjoy it. That way if it is defect out of box you can return it or replace it.
I know we tend to treasure these trains as valuable heirlooms, but lets be real. Investing in rolling stock is a loosing proposition. A nice diner out is $100 and you do not get to keep it and re eat it for a few years. Around of golf???
Sure, if you drop it and break shell or major chassis parts your out of luck. But most repairs are smoke rebuild, coupler, bulbs/leds, harnesses and circuit boards which at least for MTH are pretty general and reprogrammable. Think what happens if this occurs for Lionel.
In the end you will even have greater value in selling a dead engine for salvage parts. Just enjoy it. G