1. http://www.jsoftware.com/papers/fork.htm See Hook and Fork. This was the first introduction of Trains programming in APL, I think. You can see help.dyalog.com for more information of the current implementation in Dyalog.

2. I think the tree printout of trains is a built in library feature of Dyalog APL. The dfns library (dfns.dyalog.com) has a tree printing dfns that may be of use in implementing it, and I think you can read the actual source code for that in the user command for display or the like.

I may implement a version of this for my ASTs to augment the Xml and px utilities, as it is a nice format and one that might be of use for printing out the AST of the compiler. I have not had need of it yet, though, so I'll probably get to it the next time I have to do a fancy demo for which I can prepare more adequately.