Secret Subways music was the application of basic mathematics to melody within the framework of a given rhythm. For example, if you take a note on a keyboard, one octave up or down will be a defined distance, the next octave up will be equidistant to the first and so on. When you integrate fractions of these octaves you can bulid up complex patterns as a function of distance. Computers strength is of course calculation. Using the computer's processing power to create random melodic variations within parameters (so you don't get too many bum notes) around a given rhythmic framework (sometimes layering to create polyrythms) was how most of Secret Subway's music was created.
I know that all sounds a bit poncy and complicated, but its not really - it's just using tricks on a step editor.

Equipement - a computer and the usual software packages. (no instruments)