Here is my proposed route in more detail:
http://www.townend.me/files/hatfield1.jpg
A close up near Galleria:
http://www.townend.me/files/hatfield2.jpg
Remember I'm proposing LRVs, so sharp curves (50m radius), level crossings and short street running sections permitted (although minimised as much as possible for performance)
I have to ask, do you actually know the area or have you simply mapped a route over a Google Earth map?
The problems with it are:
The junction of the A414 / A1M is incredibly busy - there's nowhere to put a tram line and street running is completely out of the question there.
The route around the Galleria is probably not viable or would be incredibly disruptive to the Galleria itself.
There's no room around Fiddlebridge Industrial Area - you'd have to obliterate that to fit the lines through.
The 'Hatfield Centre' stop is a good 5 - 10 minute walk away from the main shopping area.
There's no space on the approach to Hatfield station to fit a further line in - with development having covered the old rail formation on Great North Road and between Beaconsfield Road and the station.
It's worth remembering that the Hatfield - St Albans line was closed before Beeching - with passenger services being withdrawn as far back as 1951, as were the Welwyn - Hertford services.
The majority of re-instatements have been of Beeching closures - I can't think of any pre-Beeching closures which have been re-opened (although I'm sure I'm about to be corrected).
The reality is the likely cost of a Light Rail scheme from St Albans to Hatfield is never likely to be viable - particularly given how well served that corridor is by the multitude of bus services which run along it.