common connections

I have three loops and my 14v accessories all on a common ground. Total wattage is about 300 (transformer capacity) but reality is at most three locos with one with passenger cars, plus 75watts of accessories and lights at one time. I have a 14 awg bus and never had a problem.

If the 4 independent loops have crossovers so you can go from one loop to the other  I would join the commons (U connections) and  phase the transformers. If the loops are completely separate it doesn't make much difference  unless  running TMCC command .Then  join the U connections.

I would run 4 separate commons to the transformer. 

Reasons:

             1 train derails the other 3 or 2 can keep running. big advantage at a train show. 3 trains can still be running. 

              2 ease of trouble shooting. 1 derailment stops everything where do you begin.

              3 better setup for command control DCS. if you have it or want to someday upgrade.

               4 in command all engines would have to be reset in case of a derailment.

               5 only one size wire needed. One common ground would have to handle the power from all 4 loops. In theory it would have to be 4 times bigger.

From an electrician's "best practices" all commons on an AC powered toy train layout should connected together.  (Of course there are exceptions to every rule.)

I agree with Susan.   I used electrical panel neutral buss bars to terminate all commons, track and accessory.  They are all hooked together.

If you're running zw's they have the four variable posts--A to D but the four U posts are already connected together internally. With the two zw's you probably should phase and connect the U outputs together.

Scotie