When you need more than one transformer to power your toy train layout there are a number of ways to hook them up. 

One way is to keep them electrically isolated from each other.  For example, use one for track power to run the locomotives and lights in cars and switches and use another for accessories, floodlights, streetlights etc.

Another way is to hook them all together with a common ground in which case they must be in “phase” with each other.

Ray Plummer (photos by William Zuback) discusses the use of multiple transformers and phasing transformers in “Postwar power basics” (October 2003 issue of Classic Toy Trains – page 50-51).

Another source of information on phasing transformers is on  

“MartyE’s Online O Gauge Train Web Page” martye.com.

Select Phasing a PWC ZW Controller with a Classic PW ZW under his Tips and Tricks Section.

This is a through discussion – with pictures — of what phasing is, why it needs to be done and how to do it.