Trainspotting Prequel Forthcoming?

 
Yes.

From a story in the Guardian...

Six more novels later, Welsh had an idea to dig those old words out again. They were stored on floppy disks he couldn't even read, so he found a data recovery expert on the internet, posted the disks off, and wondered what, if anything, would come back. "I was terrified that it would just get lost in the post. But if it was meant to be it was meant to be. I just thought: 'Oh well, I can't even remember what was on them anyway.'"

When the words returned safely, Welsh figured the next step would be straightforward. "Naively, I thought, well, I've got 100,000 words here, this should be all right," and so he sat down to write a prequel to Trainspotting. But immersing himself once again into the violent, darkly comic, chillingly affectless, uproariously chaotic world of his famous fictional junkies. Welsh ended up writing an epic so weighty and sprawling, it leaves Trainspotting looking like a footnote. "I just got into it," Welsh grins cheerfully. "It was fun. It was like meeting a bunch of old pals."

The book revisits Trainspotting's cast of Renton, Sick Boy, Begbie and co, following the group's early flirtations with heroin before they slowly descend into full-blown addiction.