I have a schengen visa issued by The Netherlands. I will be landing there first but wish to travel to a few other countries. Can I enter these countries by plane if my visa was not issued from there or just by train?
The Schengen visa is valid for the entire Schengen Area, that is indeed the purpose of the visa. There are no internal border checks within the Schengen area, so it is quite likely that nobody will even look at your visa as long as you are traveling inside the Schengen area.
Generally, a short-stay visa issued by one of the Schengen States entitles its holder to travel throughout the 26 Schengen States for up to 90 days in any 180-day period. Visas for visits exceeding that period remain subject to national procedures.
The visa applies equally to every country, EU source:
Any person, irrespective of nationality, may cross the internal borders without being subjected to border checks.
Your chosen mode of transportation makes absolutely no difference in the terms of your visa. You can fly, take a train, drive or bike across the internal Schengen borders. In short, you're completely fine.