i have just googled and found this..........
"Trainspotting" is defined in the novel's short glossary as "keeping obsessive notes on the arrival and departure of trains." People actually do this. The use of this title is probably more than a little ironic, as the rush of heroin, sex, alcohol, Iggy Pop concerts, violence, and so on, could be seen as the polar opposite to such a mundane pastime as writing down the numbers of passing trains. Then again, heroin users who are far enough along into their addictions are known to do very little (staring at walls, etc.) except for when the withdrawal symptoms begin to kick in.
.. Great Question!!