One of the smallest islands with rails - but where?

 
Fehmarn?

 

Interesting thought - I had to look than one up.  But, no, much smaller.

 
or possibly the artificial island on the Oresund bridge, Peberholm (if that counts?).

 


Denmark comprises the peninsular of Jutland, sticking out on map to the north of Germany, and around 1400 islands of which 70 are populated.  The most populous is Zealand, with a population of over 2 million.  Zealand is connected to Sweden to its east by the Oresund bridge / tunnel which starts as a tunnel and becomes a bridge on the artificial island of Peberholm.  And it is connected to the west to the island of Funen (Fyn in Danish) again on the railway again starting with a tunnel which becomes a brifge - this time on the Island of Sprogo where this picture was taken. In this case, the parallel road is on a bridge all the way  from Zealand to Funen - a suspension bridge over the shipping channel where the railway is in a tunnel, then both bridges run side by side at a much lower level to complete the crossing to Funen.

Peberholm is 130 hectares ... on the Oresund bridge / tunnel. It does count - the picture is actually Sprogo Island on the Great Belt where the railway comes out from a tunnel and carries on via a bridge, and it's 154 hectares.  https://en.wikipedia.org/wiki/Sprogø

P.S.  - Peberholm is totally artificial.  Sprogo was originally a much smaller natural island but has a big extension.