Hi ed and welcome to the gauge. I assume you are into O gauge? Just a shot here..
I believe for what you are asking, you will have to invest in track switches to swap lines, and you will need to get a DCS system so that you will have the ability to control one or more trains on the same track.
It is my opinion and my opinion only, that if you use 2 Xfmrs on the same lines you will be asking for trouble. Phasing for one will be an issue, and possible back feed from one Xfmr to the other is a possibility which may result in fatal consequences.
for example- if your 12 volt ac output from one transformer is connected to a second transformer on the 12 volt ac output side, that is not plugged in, you WILL have 120 volts ac on the male end of the plug - just electricity 101, now if this were to be plugged in, all kinds of problems could occur.