I don't think there was ever a serious question that Sanchez was under the influence of drugs or alcohol. The issue is that he was texting while trying also to drive a passenger train. Period. There may be psychological conditions that can cause this, such as Narcissistic Personality Disorder. Some discussions of that disorder say that attention is the person's "drug". That he was clearly distracted by the attention of teen railfans at least suggests this might have been part of the problem.

There’s another problem that hasn’t been followed up on: why was the Chatsworth engineer hired by Veolia with a shoplifting conviction? Some of the early coverage mentioned a response (I believe from Veolia) that a misdemeanor conviction didn’t necessarily disqualify an applicant, depending on its relation to his/her job. But the job of engineer is heavily rules based, and the tragedy occurred due directly to the inability of the engineer to follow rules – starting with a typical prohibition on the use of personal cell phones while on duty, ranging through the responsibility to call signals and to report any problem with signals to the dispatcher. A shoplifting conviction is a pretty basic indication that the individual has a problem with rules.