As you can see from the pdeaves map link above to the fabulous National Library of Scotland side by side maps, the layout north of Hatfield was 6 lines arranged as Dunstable and Luton single line, 4 track east coast main line, Hertford single line.
At one stage there were 5 platforms at Hatfield: the island platform (current 2%3) a platform opposite modern 2 where the car park now is, separated from the current platform 1 by the barrow crossing and a bay platform at the south end of platform 1, whose edging stones you can still see at the south end.
Junctions north of Hatfield were immediately north to St Albans and, as has already been mentionned, north of the current WGC station for Luton and Dunstanble (west) and Hertford (East). As we Hatfield folk know, WGC is the new kid on the block as they are only 100 years old.
All three branches are now in part cycle paths, so lots to explore if you want to: Alban Way (Hatfield to St Albans Abbey), Ayot Greenway (WGC to Wheathampstead) and the Cole Green Way (Panshanger, east WGC) to Hertford football club. Lots of bits have been built on, lest people start crayoning restoration!