When IC Controls introduced the TPC-400 (late 90s?), I bought one, powered it with a 20A transformer, and used it on a large show layout. I ran three post-war lighted passenger trains drawing over 10A. A derailment could be "interesting" with pitting on the offending metal parts - just a bit short of spot welding. That was my impetus to investigate fast circuit breakers. The TPC breakers are thermal breakers with a significant lag in tripping when shorted.
The Airpax salesman queried my order of 10A instant trip magnetic breakers stating the only application he could think of was protecting electronics downstream of the power supply. Exactly.
An application sheet for another type of reset-able breaker suggested paralleling breakers to divide the current load - the more sensitive breaker trips first and the second trips instantly thereafter. With the goal of selecting a max amperage of 10 or 20 amps, I paralleled 2 10A Airpax breakers and rigged up resistance coils to sink high amps. I found the paralleled Airpax breakers consistently tripped at 15A, not the expected 20A, for a resistance load. This suggests electrical behavior with paralleled breakers that is beyond my ability to explain.
So now we have the OP's setup pushing 20A from paralleled Powerhouses through a TPC400 that chops the AC waveform to modulate track voltage. The Powerhouses have an overcurrent protection circuit that works quickly when directly powering the track. But the TPC's Triac alters the AC current and how the Powerhouse senses a short via the TPC is an unknown. How the division of amperage between the twin Powerhouse's overcurrent circuits is also TBD.
If one of the paralleled Powerhouses is turned off, does the single unit sense the short instantly? Will the paralleled Powerhouses trip when the TPC is not in the circuit?
Inserting an Airpax instant trip magnetic breaker between the TPC and the track would be good insurance. The TPC's thermal breaker is inviting trouble since the Powerhouses are not reacting as expected.
The Airpax (under the Sensata corp) Snapak magnetic hydraulic breakers I use have a part number of PP11-69-10.0A-XX-V. They are single pole push-pull buttons (PP11) which are typically special order. The "69" portion of the part number is trip delay. "69" is instant. "61" is fast. Sensata has a configuration tool to pick the style, delay, and amperage.
The electronic suppliers show lots of Snapak breakers. When ordering, make sure the part number matches what you plugged into Sensata's configuration tool.