There is a short in the engine or tender or track. It is drawing 7-8 amps which is way too high. The Z4000 won't trip the breaker until around 8 amps.
First take the engine and tender (and any other trains) off the track and advance the throttle. With nothing on the track, the amperage should be 0 or maybe 0.1 or 0.2 AMPS. If it goes high again there is a short on the layout.
Then put the engine and then the tender one more time on the track separately to see which one is shorted. Advance the handle and power off if it goes above 2 amps. Then pull the shell off whichever one is shorting. You may be able to locate the short (pinched wire, burn marks). Do not run the engine until the short is located. So far at least some of your circuit boards have not fried, but I would not push your luck. If you cannot find it easily, send it to a tech to evaluate it.
