Things must have changed since the moment I gave up on trying to get to the islands - I was told that the ferry crossings were limited in spring time, and all I could do then was to try figuring out a way of getting there - which I could never succeed.
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VLIELAND :
Amsterdam CS -> Harlingen Haven. Dp 8 :56 Arrives at 11 :59. A 3-hour journey, 3 connexions. 2 hours of waiting time in Harlingen.
Ferry Harlingen -> Vlieland : http://tinyurl.com/3gnly4e . Ferry leaves at 14 :15 . Last ferry leaving Vlieland : 16 :45
Train Harlingen Haven -> Amsterdam CS : Dp 17 :04 Ar 20 :02.
6 hours of travelling time by train + 2 hours waiting for the ferry = 8 hours in transportation time all in all (excluding crossing time). Maximum length of stay on Vlieland : 2h30
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TERSCHELLING :
Ams CS -> Harlingen Haven : Dp 8 :56 Ar 11 :59 Harlingen -> AmsCS : Dp 16 :04 Ar 19 :03
Ferry Harlingen -> Terschelling Dp 12 :30 Last departure from the island : 17 :30
6 hours of train + ½ hour of waiting : 6h30 transportation time. For a maximum length of stay on the island : 5 hours
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SCHIERMONNIKOOG :
Train Ams CS -> Groningen : Dp 8 :56 Ar 11 :14
Bus from Groningen to Lauwersoog : Dp 11 :34 Ar 13 :10 - 2 connexions
Ferry :Dp 13 :30. Crossing time : 45 min.
So leaving Ams CS at 8 :56 -> arrival on the island at 14 :15 = 5h15min transportation.
Ferry back to Lauwersoog : last departure from Sch. : 18 :30. Arrival at Lauwersoog 19 :15
Bus from Lauwersoog to Groningen : Dp 19 :24 Ar 20 :24
Train Groningen -> Ams CS : Dp 20 :46 Ar 23 :03
So leaving the island at 18 :30, arrival in Ams CS at 23 :03 = 5 hrs
Total transportation time : 10h15 min. Maximum length of stay on the island : 6h15 min
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AMELAND
No trains to Holwerd, and www.92920v.nl does not know about the location either.
So, unless one has a rental car, or a hell of a lot of patience (to spend more time travellin to and from an island than on the island itself), or does not mind biking it all the way, I do not see it has a realistic thing to do when you are in the country for a limited time. Which I regret.