A new power-routing turnout looked perfect on the bench, but in service a four-axle switcher would occasionally arc at the frog — a visible spark and a momentary command-station short-trip — and six-wheel-truck diesels would stall dead on the frog at crawl speed. Longer-wheelbase steam rolled through fine.
Two symptoms, likely two mechanisms. The stall is a dead-frog problem: a plastic (unpowered) frog leaves a gap longer than the loco’s pickup wheelbase, so a short-wheelbase loco briefly has no powered wheel — it coasts, and at crawl speed it stops. The arc is the opposite: a powered metal frog whose polarity didn’t match the route the points were set for, so the first wheel to bridge railhead-to-frog shorted the two rails. A meter across the frog and the stock rails, toggled with the points, confirmed the frog polarity was static — it never flipped with the turnout.
A power-routing turnout’s frog must be the polarity of whichever route is lined. If it is hard-wired (or left to a worn point-contact that no longer switches cleanly), then for one of the two routes the frog is the wrong polarity. A wheel tread spanning the frog and the adjacent stock rail then connects + to − directly — a bolted short across the bus. The command station trips (or arcs if the trip is slow), and the pitting from each arc makes the next contact worse.
The stall is the companion failure: without a reliably powered frog, you are relying on the loco’s pickup span to bridge an electrically dead zone. Short-wheelbase locos lose the bet.
Power the frog and switch its polarity with the route:
- Drive the frog from a polarity source that follows the points — a switch-machine auxiliary contact, a frog-juicer (auto-reverse-style polarity sensor), or a DPDT tied to the throw. Now the frog is always correct for the lined route; no short, no dead zone.
- Gap the frog rails (insulated joiners on both frog-side rails) so the powered frog can’t back-feed the wrong polarity into the next section.
- Dress the arc pitting on the frog before it becomes an intermittent-contact source of its own.
At a metal-frog turnout, “arcs” and “stalls” are two faces of one question: is the frog powered to match the route? Make the frog polarity follow the points and both failures disappear at once.