diff --git a/worker/src/jobs/build-valhalla.ts b/worker/src/jobs/build-valhalla.ts
index 581e23d..461339d 100644
--- a/worker/src/jobs/build-valhalla.ts
+++ b/worker/src/jobs/build-valhalla.ts
@@ -95,6 +95,106 @@ function runProcess(cmd: string, args: string[]): Promise<void> {
   });
 }
 
+class ProcessError extends Error {
+  constructor(message: string, public readonly lines: string[], public readonly signal: string | null) {
+    super(message);
+  }
+}
+
+/** Like runProcess but captures stdout+stderr (still printed) for error analysis. */
+function runProcessCapture(cmd: string, args: string[]): Promise<void> {
+  return new Promise((resolve, reject) => {
+    console.log(`[build-valhalla] Running: ${cmd} ${args.join(" ")}`);
+    const child = spawn(cmd, args, { stdio: ["ignore", "pipe", "pipe"] });
+    const lines: string[] = [];
+    const onData = (d: Buffer) => {
+      process.stdout.write(d);
+      lines.push(...d.toString().split(/\r?\n/).filter((l) => l.length > 0));
+    };
+    child.stdout!.on("data", onData);
+    child.stderr!.on("data", onData);
+    child.on("error", reject);
+    child.on("exit", (code, signal) => {
+      if (code === 0) resolve();
+      else reject(new ProcessError(
+        `${cmd} exited with code ${code}${signal ? ` (signal: ${signal})` : ""}`,
+        lines, signal,
+      ));
+    });
+  });
+}
+
+// ─── Transit stop quarantine ───────────────────────────────────────────────────
+
+/** Path to the quarantine file (outside feed/ so it survives re-filters). */
+function quarantineFilePath(citySlug: string): string {
+  return `${GTFS_DATA_DIR}/${citySlug}/.quarantine`;
+}
+
+function readQuarantine(citySlug: string): [number, number][] {
+  const p = quarantineFilePath(citySlug);
+  if (!existsSync(p)) return [];
+  try { return JSON.parse(readFileSync(p, "utf8")) as [number, number][]; } catch { return []; }
+}
+
+/** Append new bad coordinates to the quarantine file. */
+function appendQuarantine(citySlug: string, badCoords: [number, number][]): void {
+  const merged = [...readQuarantine(citySlug), ...badCoords];
+  writeFileSync(quarantineFilePath(citySlug), JSON.stringify(merged));
+  console.log(`[build-valhalla] Quarantine updated for ${citySlug}: ${merged.length} stop(s) total`);
+}
+
+/**
+ * Remove quarantined stops from the city's filtered GTFS feed (stops.txt + stop_times.txt).
+ * Called before valhalla_ingest_transit so the bad stops never enter the transit graph.
+ */
+function applyQuarantine(citySlug: string): void {
+  const badCoords = readQuarantine(citySlug);
+  if (badCoords.length === 0) return;
+
+  const feedDir = cityGtfsFeedDir(citySlug);
+  const stopsPath = `${feedDir}/stops.txt`;
+  if (!existsSync(stopsPath)) return;
+
+  const stopLines = readFileSync(stopsPath, "utf8").split(/\r?\n/).filter((l) => l.trim());
+  if (stopLines.length < 2) return;
+  const headers = stopLines[0].split(",").map((h) => h.trim().replace(/^\uFEFF/, ""));
+  const stopIdCol = headers.indexOf("stop_id");
+  const latCol    = headers.indexOf("stop_lat");
+  const lonCol    = headers.indexOf("stop_lon");
+  if (stopIdCol < 0 || latCol < 0 || lonCol < 0) return;
+
+  const removedIds = new Set<string>();
+  const keptLines  = [stopLines[0]];
+  for (let i = 1; i < stopLines.length; i++) {
+    const fields = stopLines[i].split(",");
+    const lat = parseFloat(fields[latCol] ?? "");
+    const lon = parseFloat(fields[lonCol] ?? "");
+    if (badCoords.some(([bLat, bLon]) => Math.abs(lat - bLat) < 1e-5 && Math.abs(lon - bLon) < 1e-5)) {
+      removedIds.add(fields[stopIdCol] ?? "");
+    } else {
+      keptLines.push(stopLines[i]);
+    }
+  }
+  if (removedIds.size === 0) return;
+
+  console.log(`[build-valhalla] Quarantine: removing stop(s) ${[...removedIds].join(", ")} from feed`);
+  writeFileSync(stopsPath, keptLines.join("\n") + "\n");
+
+  const stPath = `${feedDir}/stop_times.txt`;
+  if (!existsSync(stPath)) return;
+  const stLines = readFileSync(stPath, "utf8").split(/\r?\n/).filter((l) => l.trim());
+  if (stLines.length < 2) return;
+  const stHeaders = stLines[0].split(",").map((h) => h.trim());
+  const stStopCol = stHeaders.indexOf("stop_id");
+  if (stStopCol < 0) return;
+  const stOut = [stLines[0]];
+  for (let i = 1; i < stLines.length; i++) {
+    if (!removedIds.has(stLines[i].split(",")[stStopCol] ?? "")) stOut.push(stLines[i]);
+  }
+  writeFileSync(stPath, stOut.join("\n") + "\n");
+}
+
 function buildTimezoneDb(): Promise<void> {
   return new Promise((resolve, reject) => {
     console.log("[build-valhalla] Running: valhalla_build_timezones → " + TIMEZONE_SQLITE);