J Daddy;
Fastest help is going to be Run less train at any one time or use a Much Bigger Transformer
Then running a new heavy wire power drop to the middle of any area with low volts.
And I agree with gunrunnerjohn, Converting lighting to LEDs will reduce your power needs dramatically so you don't need a bigger transformer.
The Drop to 13 volts shows you are overloading the transformer, or nearly doing so.
You will always have loss in the track, but it should not be more than a couple volts at any point.
If you want to run several trains with lots of standard lighted cars, you will need the Biggest Transformer you can get or one medium to big Transformer for each loop or block.
