Running 2 PS2 Trains on Same Track Blows Fuse

What size fuse?  What locos? When you say "fuse trips on my Z4000" do you mean the internal breaker, the breaker on the rear, or do you a fuse at the Z4000?  Does it matter which loco you move first?  Is smoke on or off?  What do ammeters on the Z4000 read when this happens?

Is this locomotives only or are lighted passenger cars involved?

Also, I've encountered [too many] diesels where the wire(s) from the truck(s) is/are too close to the frame and the insulation gets shaved. It creates what I "affectionately" call a "resistive short" where instead of a dead short, there's a high current draw -- often enough to trip a breaker if another train rolls into the block. If you have a diesel that draws more than 1.5 amps, you should remove the shell and check the red wires coming in from the trucks. A simple fix is electrical tape on the holes in the chassis, but you're better off removing the truck and shrink-wrapping the wire.

I run a premier proto2 Santa Fe ABA with 6 passenger cars on the same track with a proto2 premier GG1 and 5 passenger cars and have no fuse issues.  Got to be more happening.

Tried again when  get home and all seems to be working fine now...one of the engines may not have been on the track correctly ....that's the only think I can think of....ty for the responses...

I'd still check those pickup roller wires. The resistive short situation is intermittent. A train with a frayed hot lead can run for hours without incident, then suddenly the locomotive is drawing 6 amps by itself.