I can't speak for MTH passenger cars as I only have K-Line and GGD cars in 3 rail. However at my club layout, with the GGD cars with LED illumination, I can run two powered MTH engines and around 18-20 cars at 7-8 amps on a 2% max grade under command. That includes 4 K-line incandescent lit club cars. With all K-Line cars my limit is about 10-12. I plan on relighting them all with LEDS as my Daylight set is 16 cars long.
Currently I am experimenting on my home layout and I have 4 powered 3rd Rail locomotives pulling 15 scale cars with two being K-Line. It runs right at about 9 amps. Yes, the number of locos is overkill, the but sound of four units at once is awesome!
I should point out that I don't run smoke. That may help my energy budget.