Sweden
In Sweden all electric railways use 15 kV 16 2/3 Hz AC. Except that two local isolated railways, Saltsjöbanan and Roslagsbanan use DC power. Also the Stockholm Metro and tramways use DC power. The Oresund Bridge going to Denmark use 25 kV, Danish standard. The border between the voltages is on Swedish land near the bridge. Trains pass this point at speed. Only two-voltage trains (or diesel trains; rare) can pass the point.
Read more about this topic: 15 KV AC Railway Electrification