The Baltic, and Bornholm, indeed correct -- as stated, the island much nearer to Sweden (and closer to Poland and to Germany, than to the nearest point in the rest of Denmark). Gauge, indeed a metric one -- viz. 1 metre. Unlike the other Scandinavian nations, Denmark was never a big-time narrow-gauge country: such n/g as it had, was nearly all metre-gauge; including Bornholm, whose system lingered on for several decades longer than any metre-gauge elsewhere in Denmark. Bornholm's system had at maximum, three lines: two closed in the early '50s -- the final one (Ronne to Nekso, western end of the island, to eastern) lasted till 1968.
Thus, no-one's suggestion re the gauge, got the fully right one;
@DerekC , you were on the money with all else: so, your floor.