The plug that goes into the wall socket, on any generic replacement, must have all three contacts; the two blades for power, and the pin for ground.
That ground pin must connect to earth ground in your house because it makes up half of the antenna for TMCC/Legacy (the other half being the outside rails on the layout).
99.9% of transformers (i.e., wall worts) do not have this.
There's a work around though, involving a short jumper that you would have to fabricate for the output side of your transformer in order to make a connection to that earth ground.
Fellow forumite @ADCX Rob suggests a suitable replacement power supply in this post, featuring screw terminals, which make the work-around a little easier (you can then follow the thread after his post and see exactly how the jumper is made):
All Electronics ACTX-12167 Screw Terminal Wall Transformer | ADCX Rob
Mike
