I have continuous run layout in a 14' x 14' room (folded dogbone), wired in blocks for DC, but with a DPDT toggle on the control panel so I can switch to DCC, with the appropriate decoder-equipped locomotives, But I have recently started encountering problems with the DC power packs tripping their internal thermal circuit breakers after I run a pair of Athearn "blue box" GP-7's for a few minutes. I have an analog ammeter on my control panel which is showing only 1-1/2 amp Because of the track layout, which has what is effectively a reverse loop with manually operated DPDT switch to match polarities at either end, I first thought I might have a short circuit somewhere. But I shut off all the blocks, and turned them on one at a time, running one of four GP-7's one at a time, and...NO PROBLEM! But run two of them in any combination and after a couple of minutes, the breaker pops. Turn the power off for a couple of minutes until the pack cools down, and try it again with two units, and the same thing happens! I have a fifth GP-7 with a non-sound decoder, and can run it separately almost indefinitely on DC. Switching to DCC, this unit shows NO SIGN of any short circuit. I have a MRC Tech 4 280, which has a fairly good capacity. Thinking perhaps that this unit, which is a few year old, might be becoming sensitive, I obtained some slightly lower capacity packs, a Tech II and a 1380. Same thing! I'm baffled. Don't recall having this problem with two powered unit before. Any ideas?