Sundsvall Central Station serves trains, not aircraft, so a search for a lounge at this address is really a search in the wrong place: this is Sundsvall's main railway station on Sweden's northern rail line, not an airport, and it has no airport-style lounge because it was never built to be one, whatever specific membership or airline program prompted the question in the first place.
The city's actual air gateway is Sundsvall-Timra Airport, a regional facility a short drive from the center that handles domestic routes, mainly connections to Stockholm, and lounge provision at an airport this size is limited by nature. Do not expect an elaborate premium space there; check directly with your airline whether any lounge or priority service currently exists before you count on it, since regional Swedish airports update these details less frequently than the country's main international hub does.
For a fuller lounge picture, Stockholm is the realistic step up. Stockholm Arlanda Airport, reachable from Sundsvall by a short domestic flight or a longer train journey south, is Sweden's principal international hub and carries a substantially larger network of airline and membership lounges than a regional airport like Sundsvall-Timra can support, reflecting its role handling the bulk of Sweden's international connections to the rest of Europe and beyond.
For travelers actually passing through Sundsvall Central Station, the practical picture is ordinary station amenities, seating, food options, and connections to regional and long-distance trains along Sweden's northern line, rather than anything resembling an airport lounge. If your onward journey continues by air, route your lounge expectations toward Sundsvall-Timra for a limited regional option or Stockholm Arlanda for the fuller network, and confirm your specific airline or membership program's current terms at whichever airport your trip actually uses, well before you head to the station or the gate on travel day.