It seems that although the emissions from construction for railways is more than twice as large as that from the construction of airports, on a passenger-kilometer basis, the emissions from the flights themselves dwarfs those of trains no matter what fuel source are used for the trains. The following table is from Sweden where there is a relatively large portion of hydropower used to power the trains, but it is hard to imagine any fuel mix exceeding those of flights:

[![Table showing Life Cycle emissions of various modes of transportation, broken down by construction and use][1]][1]

The figure can be found in [this report from Swedish IVA][2], and is based on the freely availble (although in Swedish) calculator found [here][3].

This answer also debunks the answer to [this question][4] and I would be happy if anyone could link to this answer from that question (I am short on points to do so myself).


  [1]: https://i.sstatic.net/5nu3l.png
  [2]: https://www.iva.se/globalassets/rapporter/ett-energieffektivt-samhalle/201411-iva-energieffektivisering-rapport9-english-d.pdf
  [3]: http://www.trafikverket.se/tjanster/system-och-verktyg/Prognos--och-analysverktyg/Klimatkalkyl/
  [4]: https://skeptics.stackexchange.com/questions/38122/is-flying-the-worst-choice-of-travelling-considering-the-greenhouse-effect-and/38179#38179