I just learned about something called Silverpilen - a ghost train in Stockholm. That would be an interesting Mystery for the kids. What other ghost stories might work for a horror-themed campaign? Might be fun to do something scary for Halloween.
Story hook: there are rumours of a secret extension of the Green Line at the end station at Hässelby strand, connecting the Loop with the Stockholm metro. The station itself is above ground, but an unused (?) track leads to a locked gate. If there's such a tunnel, it's about 8 km (5 miles) long. Has the mysterious "Silver Arrow" train any connection with it, and what about reports of strange creatures emerging from service hatches and air vents at Hässelby strand?
Hässelby strand's location on the Mälarö map:
My life fades. The vision dims. All that remains are memories. I remember a time of chaos, ruined dreams, this wasted land.