What is the meaning of the title Trainspotting?

From Trainspotting (film): Legacy comes:

The cryptic film title is a reference to a scene (not included in the film) in the original book, where Begbie and Renton meet 'an auld drunkard' who turns out to be Begbie's estranged father, in the disused Leith Central railway station, which they are using as a toilet. He asks them if they are "trainspottin'."

Irvine Welsh (book author) himself has explained in a Q&A that the title is also a reference to people thinking that the hobby of trainspotting¹ makes no sense to non-participants. Likewise, the same applies to heroin addiction: to non-addicts the act seems completely pointless whereas, to someone hooked on heroin, it makes absolute sense.

1. As to the hobby of trainspotting:

   ..Those who are "trainspotters" make an effort to "spot" all of a certain type of rolling stock. This might be a particular class of locomotive, a particular type of carriage or all the rolling stock of a particular company. To this end, they collect and exchange detailed information about the movements of locomotives and other equipment on the railway network, and become very knowledgeable about its operations. ..

TRANSPOTTING Originally it refers to the hobby of spotting and keep track of trains.

CAMBRIDGE DICTIONARY: TRAINSPOTTING - the activity of watching trains and writing down the numbers that each railway engine has http://dictionary.cambridge.org/dictionary/british/trainspotting

In a wider sense, trainspotter might refer to someone who is obsessed with something that appears to be completely meaningless to an outsider. In this case the hobby might be exercised to an extreme extent, completely overshadowing the person’s life and thus making the obsession more important than anything else in life (like education, family, career, friends, health and well being and so on).

OXFORD DICTIONARY: TRAINSPOTTER - often derogatory A person who obsessively studies the minutiae of any minority interest or specialized hobby: ‘the idea is to make the music really really collectable so the train spotters will buy it in their pathetic thousands’ http://www.oxforddictionaries.com/definition/english/trainspotter#trainspotter__5

It could be about anything really; a collection of beer cans, stamps or Marilyn Monroe memorabilia. The behaviour resembles in some manners the compulsive behaviour and obsessed fixation of a heroin addict forced on by physical abstinence, which by the same standards makes little or no sense to anyone else: "Who needs reasons when you got heroin?"

That's about it! This is my interpretation as far as it goes, and I might have missed out on some details. Sorry about that, but I'm not a trainspotter... ;-)

I think the obvious reference I can't seem to find elsewhere here, is that Heroin users look for veins or arteries to shoot dope into and end up with "track marks". So Heroin users look at tracks and for spots to inject themselves.