Is a research article the most comprehensive information available about a research project? I sometimes stumble upon an article about an experiment, e.g a medical experiment done in some medical journal, or an article refering to a physics experiment done, and would like to know more exact details about the experiment and just in general as much information as possible about the subject.
I never wrote an article for a journal nor have I participated in the management of an experiment, but I understand documentation is done beyond what is written in the article, a journal article is a summary of the experiment and only elaborates on the important points.
For example a lab book AFAIK is often kept and used during the course of an experiment.
But this question pertains to theoretical research as well, e.g in mathematics; I may wish to find more information about the process by which the author came to his findings.

Is there usually a more complete documentation available which delineates the whole process of the experiment/research-project?
One thing I found is this Wikipedia article about "Open notebook science", but I understand this is not the norm. Still, I suspect I may be missing something (maybe a iece of documentation not as comprehensive as a lab notebook, but more so then a journal article. 
Is there anything more for me to do if I wish to dig deeper and understand the subject more thoroughly?
Is there usually a more complete documentation available which delineates the whole process of the experiment/research-project?


Typically not all of the details of the research can feasibly be published. Different areas have different methods to cope with this. Some journals will allow supplementary material to be submitted that's made available online through the publisher's site. Some authors will add links to homepages for the project where more details are available. Sometimes authors will publish an extended "technical report" (e.g., with full proofs or full methodologies that would not be feasible under a page limit) and publish it on their institute homepage or a site like arXiv. I believe some biomedical journals allow for publishing databases containing micro-data on experiments ...

So it varies widely. The most generic answer, however, is simply to get in contact with the authors and ask them if more details are available! Their contact details should be provided in the paper.