Regulating the power inside the engine is known as cruise control. 
However Rich is right, fix the track.
When I first laid my track for my new layout, I did a quick test by connecting power at one point on the 12 x 24 layout and running an engine around the whole layout. I had no issues running, though when I actually measured the voltage at the far end under a 2A load, they varied about a volt and a half from the connected transformer voltage, not bad for all that track and joints.
FWIW, I now have probably at least 75-80 power drops around the whole layout with 500 feet of track in total.
