Adding to house earth ground

There should be earth ground at every outlet and light in your house. If not, you should have an electrician check and see what is going on.  Since the earth ground does not carry any current, except under fault conditions, wire size should not be a problem. You are putting the signal onto the earth ground from the ground wire from your transformer. The location of the actual connection to the ground rod is not an issue. 

I have heard of some people putting an earth ground grid in the ceiling above their trains. If you are having signal trouble, it would not hurt to check to be sure that the ground wire from the transformer is actually connecting to the ground wire in the outlet. There are circuit checkers that can be bought at home improvement stores that will check to be sure that your outlets are properly wired and the ground is connected.  You just plug them into the outlet and a certain combination of three little lights should come on.  That maybe a place to start. 

One misconception is that for TMCC to work properly, there must be a ground wire that actually goes into the earth.

This is not true.

The third prong on the TMCC base power plug is the so-called "earth ground." But really its just an antenna. And for it to work as an antenna it does not have to go into the earth, nor does it have to be connected to your house wiring.

It only has to be long enough to act as a good antenna be and close enough to your layout for your trains to receive the signal.

You could use a long extension cord as your "earth ground" antenna.

Plug your TMCC base power plug into the extension cord. Run the extension cord on the floor around the perimeter of your layout and plug it into the wall.

That should be more than enough antenna for your trains to work properly.

I've run my TMCC trains outside, using a 12 volt car battery and 120VAC inverter. No house wiring anywhere nearby and not a single wire went into the actual earth.